Exhibit
10.1 


EXECUTION
VERSION


 


SPONSOR
SUPPORT AGREEMENT


This
Sponsor Support Agreement (this “Sponsor Agreement”) is dated as of September 25, 2022, by and among GX Sponsor II
LLC, a Delaware limited liability company (“Sponsor Holdco”), the Persons set forth on Schedule I hereto (together
with the Sponsor Holdco, each, a “Sponsor” and, together, the “Sponsors”), GX Acquisition Corp.
II, a Delaware corporation (“GX”), and NioCorp Developments Ltd., a company incorporated under the laws of the Province
of British Columbia (the “Company”).


RECITALS


WHEREAS,
as of the date hereof, the Sponsors collectively are the holders of record and the “beneficial owners” (within the meaning
of Rule 13d-3 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”)) of 7,500,000 shares of GX
Class B common stock (“GX Common Stock”) and 5,666,667 GX private placement warrants (“GX Warrants”)
in the aggregate as set forth on Schedule I attached hereto;


WHEREAS,
contemporaneously with the execution and delivery of this Sponsor Agreement, GX, the Company and Big Red Merger Sub Ltd, a Delaware corporation
and a direct, wholly owned subsidiary of the Company (“Merger Sub”), entered into a Business Combination Agreement
(as amended or modified from time to time, the “Business Combination Agreement”) pursuant to which, among other transactions,
(i) Merger Sub will merge with and into GX, with GX surviving the merger, (ii) the Company will purchase all shares of Class A common
stock of GX as the surviving corporation of such merger (other than the shares of Class A common stock then held by the Company) in exchange
for Company Common Shares, (iii) all shares of GX Class A common stock will be transferred by the Company to 0896800 B.C. Ltd., a company
organized under the laws of the Province of British Columbia and a direct wholly owned subsidiary of the Company (“Intermediate
Holdco”) in exchange for additional shares of Intermediate Holdco, and (iv) Elk Creek Resources Corporation, a Nebraska corporation
and a direct wholly owned subsidiary of Intermediate Holdco, will merge with and into GX, with GX surviving (collectively, and together
with the other transactions contemplated by the Business Combination Agreement and the Ancillary Agreements, the “Transactions”);
and


WHEREAS,
as an inducement to GX and the Company to enter into the Business Combination Agreement and to consummate the Transactions, the parties
hereto desire to agree to certain matters as set forth herein.


AGREEMENT


NOW,
THEREFORE, in consideration of the foregoing and the mutual agreements contained herein, and intending to be legally bound hereby, the
parties hereto agree as follows:






    	 	 	 



    	 



    

 


ARTICLE
I 


Section
1.1      Definitions. Capitalized terms used but not otherwise defined herein shall have the respective
meanings ascribed to such terms in the Business Combination Agreement. When used in this Sponsor Agreement, the following terms in all
of their tenses, cases and correlative forms shall have the meanings assigned to them in this Section 1.1 or elsewhere in
this Sponsor Agreement.


“Certificate
of Incorporation” means the Amended and Restated Certificate of Incorporation of GX, dated as of March 17, 2021, as may be
amended from time.


“Change
in Control” means:


(i)       the
acquisition by any individual, entity or group (within the meaning of Section 13(d)(3) or 14(d)(2) of the Exchange Act) (a “Person”)
of beneficial ownership (within the meaning of Rule 13d-3 promulgated under the Exchange Act) of 40% or more of either (A) the then outstanding
shares of capital stock of the Company (the “Outstanding Company Shares”), or (B) the combined voting power of the
then outstanding voting securities of the Company entitled to vote generally in the election of directors (the “Outstanding
Company Voting Securities”), in each case, taking into account any options or other rights to acquire Outstanding Company Shares
or Outstanding Company Voting Shares held by such Person; or


(ii)       individuals
who, as of immediately following the First Merger Effective Time and the appointment of the Closing Directors and Officers pursuant to
Section 1.6 of the Business Combination Agreement, constitute the board of directors of the Company (the “Incumbent Board”)
cease for any reason to constitute a majority of such board of directors; provided, however, that any individual becoming
a director of the Company or its successor subsequent to the date hereof whose election, or nomination for election by the Company shareholders,
was approved by a vote of a majority of the directors of the Company then comprising the Incumbent Board shall be considered as though
such individual were a member of the Incumbent Board, but excluding, for this purpose, any such individual whose initial assumption of
office occurs as a result of either an actual or threatened election contest or other actual or threatened solicitation of proxies or
consents by or on behalf of a Person other than the Company board of directors; or


(iii)       consummation
of an extraordinary transaction at the Company (including by way of reorganization, merger, amalgamation or consolidation), with or without
approval by the shareholders of the Company, as applicable, in each case, unless, following such transaction, (A) more than 60% of the
then outstanding shares of common stock (or equivalent security) of the company resulting from such transaction and the combined voting
power of the then outstanding voting securities of such company entitled to vote generally in the election of directors is then beneficially
owned, directly or indirectly, by all or substantially all of the individuals and entities who were the beneficial owners of such securities
immediately prior to such transaction in substantially the same proportions as their ownership immediately prior to such transaction,
and (B) a majority of the members of the board of directors of the company resulting from such transaction were members of the Incumbent
Board at the time of the execution of the initial agreement providing for such reorganization, merger, amalgamation or consolidation;
or




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(iv)       consummation
of a sale or other disposition of all or substantially all the assets of the Company to an entity that does not control, is not controlled
by, and is not under common control with, the Company, with or without approval by the shareholders of the Company; or


(v)       any
other transaction whereby any Person obtains control over the direction of the affairs of the Company. For purposes of this definition,
“control” (including, with correlative meanings, the terms “controlling,” “controlled by” and “under
common control with”), as used with respect to any Person, means the possession, directly or indirectly, of the power to direct
or cause the direction of the management or policies of such Person, whether through the ownership of voting securities, by agreement
or otherwise.


“Permitted
Transfer” means, prior to the Closing, a Transfer (a) in connection with redemption de-risking instruments (including, for
the avoidance of doubt, to incentivize holders of Class A Shares not to elect to redeem all or a portion of such holders’ Class
A Shares, as determined in accordance with the Certificate of Incorporation and the Trust Agreement), to raise capital or to pay for
or otherwise reduce Advisor (as defined below) or other expenses in connection with the Transactions, as determined in a Sponsor’s
sole discretion, provided, that such a Transfer will be allocated (i) 50% from Vested Shares, (ii) 25% from Tranche 1 Earnout
Shares (as defined below) and (iii) 25% from Tranche 2 Earnout Shares (as defined below), provided, further, that upon
transfer or reissuance of any corresponding Class A Shares or Company Common Shares, such shares shall become fully vested and freely
transferrable and not subject to any price-based vesting or other transfer restrictions applicable to Earnout Shares (as defined below),
in each case subject to applicable law and Article V of the Amended and Restated Registration Rights Agreement, by and among GX, Sponsor
Holdco and the NioCorp Holders (as such term is defined therein), in substantially the form attached as Exhibit A to the Business Combination
Agreement (the “Registration Rights Agreement”); (b) to GX’s officers or directors, any affiliate or family
member of any of GX’s officers or directors; (c) in the case of a Person who is not an individual, to any affiliate of such Person
or to any member(s) of such Person or any of their affiliates; (d) in the case of an individual, to a member of such individual’s
immediate family or to a trust, the beneficiary of which is a member of such individual’s immediate family, an affiliate of such
individual or to a charitable organization; (e) in the case of an individual, by virtue of Laws of descent and distribution upon death
of such individual; (f) in the case of an individual, pursuant to a qualified domestic relations order; or (g) by virtue of the laws
of the State of Delaware or the Sponsor’s limited liability company agreement upon dissolution of the Sponsor; provided,
however, that, prior to and as a condition to the effectiveness of any Permitted Transfer described in clauses (a) through (g),
the transferee shall have executed and delivered to the Company a joinder or counterpart of this Sponsor Agreement pursuant to which
such transferee shall be bound by all of the applicable terms and provisions of this Sponsor Agreement.


  “VWAP”
means, for any security as of any date(s), the dollar volume-weighted average price for such security on the principal securities exchange
or securities market on which such security is then traded during the period beginning at 9:30:01 a.m., New York time, and ending at
4:00:00 p.m., New York time, as reported by Bloomberg through its “HP” function (set to weighted average) or, if the foregoing
does not apply, the dollar volume-weighted average price of such security in the over-the-counter market on the electronic bulletin board
for such security during




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the period beginning at 9:30:01 a.m., New York time, and ending
at 4:00:00 p.m., New York time, as reported by Bloomberg, or, if no dollar volume-weighted average price is reported for such security
by Bloomberg for such hours, the average of the highest closing bid price and the lowest closing ask price of any of the market makers
for such security as reported by OTC Markets Group Inc. If the VWAP cannot be calculated for such security on such date(s) on any of
the foregoing bases, the VWAP of such security on such date(s) shall be the fair market value per share on such date(s) as reasonably
determined by the Company.


ARTICLE
II

SPONSOR SUPPORT AGREEMENT; COVENANTS


Section
2.1           
No Transfer. During the period commencing on the date hereof and ending on the earliest of (a) the Second Merger Effective Time,
(b) such date and time as the Business Combination Agreement has been terminated in accordance with its terms (the earlier of (a) and
(b), the “Support Expiration Time”), and (c) the liquidation of GX, each Sponsor shall not (i) sell, offer to sell,
contract or agree to sell, hypothecate, pledge, grant any option to purchase or otherwise dispose of or agree to dispose of (each, a
“Transfer”), directly or indirectly, file (or participate in the filing of) a registration statement with the SEC
(other than the Joint Proxy Statement and Form S-4) or establish or increase a put equivalent position or liquidate or decrease a call
equivalent position within the meaning of Section 16 of the Exchange Act, with respect to any shares of GX Common Stock or GX Warrants
owned by such Sponsor, (ii) enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic
consequences of ownership of any shares of GX Common Stock or GX Warrants owned by such Sponsor, or grant or enter into any proxy (other
than in accordance with this Sponsor Agreement), voting trust or other agreement or arrangement with respect to the voting of any GX
Common Stock owned by such Sponsor, or (iii) publicly announce any intention to effect any transaction specified in clause (i) or (ii);
provided, however, that the foregoing restrictions shall not apply to a Permitted Transfer. Any Transfer of any GX Common
Stock or GX Warrants in violation of any provision of this Sponsor Agreement shall be void ab initio and of no force or effect.


Section
2.2           
New Shares. In the event that (a) any shares of GX Common Stock, GX Warrants or other equity securities of GX are issued to a
Sponsor after the date of this Sponsor Agreement pursuant to any stock dividend, stock split, recapitalization, reclassification, combination,
exchange of shares or other similar event of GX Common Stock or GX Warrants of, on or affecting the shares of GX Common Stock or GX Warrants
owned by such Sponsor or otherwise, (b) a Sponsor purchases or otherwise acquires beneficial ownership of any shares of GX Common Stock,
GX Warrants or other equity securities of GX after the date of this Sponsor Agreement, or (c) a Sponsor purchases or otherwise acquires
the right to vote or share in the voting of any shares of GX Common Stock or other equity securities of GX after the date of this Sponsor
Agreement (such shares of GX Common Stock, GX Warrants or other equity securities of GX, collectively the “New Securities”),
then such New Securities acquired or purchased by, or otherwise issued to, such Sponsor shall be subject to the terms of this Sponsor
Agreement to the same extent as if they constituted the shares of GX Common Stock or GX Warrants owned by such Sponsor as of the date
hereof.



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Section
2.3          
Closing Date Deliverables. On the Closing Date, the Sponsor Holdco and the NioCorp Holders shall deliver to GX and the Company
a duly executed copy of the Registration Rights Agreement.


Section
2.4           
Sponsor Agreements.


(a)         
At any meeting of the shareholders of GX, however called, or at any adjournment thereof, or in any other circumstance in which the vote,
consent or other approval of the shareholders of GX is sought, each Sponsor shall (i) appear at each such meeting or otherwise cause
all of its shares of GX Common Stock to be counted as present thereat for purposes of calculating a quorum and (ii) vote (or cause to
be voted), or execute and deliver a written consent (or cause a written consent to be executed and delivered) covering, all of its shares
of GX Common Stock:


(i)     
in favor of each GX Resolution and any other matter necessary for the consummation of the Transactions;


(ii)     
against any (i) merger agreement, merger or other Business Combination Proposal (in each case, other than the Business Combination Agreement
and the Transactions) or (ii) consolidation, combination, sale of substantial assets, reorganization, recapitalization, dissolution,
liquidation or winding up of or by GX;


(iii)     
against any change in the business, management or board of directors of GX (other than in connection with the GX Resolutions); and


(iv)     
against any proposal, action or agreement that would (A) impede, frustrate, prevent or nullify any provision of this Sponsor Agreement,
the Business Combination Agreement or the Transactions, (B) result in a breach of any covenant, representation, warranty or any other
obligation or agreement of GX under the Business Combination Agreement, (C) result in any of the conditions set forth in Article VI of
the Business Combination Agreement not being fulfilled, or (D) change in any manner the dividend policy or capitalization of, including
the voting rights of any class of capital stock of, GX (other than in connection with the GX Resolutions).


(b)         
Each Sponsor agrees that, prior to the Support Expiration Time, such Sponsor shall not elect to cause GX to redeem any GX Common Stock
(including GX Common Stock that constitutes New Securities) beneficially owned or owned of record by such Sponsor or submit any such
GX Common Stock for redemption in connection with the Transactions.


Section
2.5           
Vested Shares; Earnout Shares; Released Earnout Shares.


(a)         
General. Each Sponsor agrees that (i) the shares of GX Common Stock held by such Sponsor set forth opposite such Sponsor’s
name as Vested Shares on Schedule




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I attached hereto, together with (A) all Second Merger Surviving Company Shares issued with respect thereto in the Second Merger, as
thereafter may be adjusted pursuant to the terms of the Exchange Agreement, and (B) all Company Common Shares issued upon exchange thereof
pursuant to and in accordance with the Exchange Agreement, to be entered into on the Closing Date in connection with the Transactions,
by and among GX, the Sponsor Holdco and the Company (the “Exchange Agreement”), shall be fully vested at the Closing
(the “Vested Shares”) and (ii) the shares of GX Common Stock held by such Sponsor set forth opposite such Sponsor’s
name as Earnout Shares on Schedule I attached hereto, together with all Second Merger Surviving Company Shares issued with respect
thereto in the Second Merger, as thereafter may be adjusted pursuant to the terms of the Exchange Agreement, shall constitute “Earnout
Shares”.


(b)         
Certain Restrictions. During the Earnout Share Period, (A) Earnout Shares that do not constitute Released Earnout Shares (or the
right to receive Company Common Shares with respect thereto) may be transferred only to a transferee described in section (a)(i) of the
definition of Permitted Transferees in the Registration Rights Agreement (regardless of whether the Lock-up Period, as defined therein,
has expired) (such transferee, a “Restricted Permitted Transferee”), (B) the applicable Exchanging Shareholder shall
not (i) enter into any swap or other arrangement that transfers, in whole or in part, to another that is not a Restricted Permitted Transferee
any of the economic consequences of ownership of any Earnout Shares that do not constitute Released Earnout Shares owned by such Sponsor
(or the right to receive Company Common Shares with respect thereto) or (ii) publicly announce any intention to effect any transaction
specified in clause (B)(i) or any transfer of Earnout Shares that do not constitute Released Earnout Shares owned by such Sponsor (or
the right to receive Company Common Shares with respect thereto) to a transferee that is not a Restricted Permitted Transferee. Without
limiting the foregoing, for the avoidance of doubt, all shares of GX Common Stock (including Vested Shares and Released Earnout Shares)
constitute Lock-Up Shares (as defined in, and pursuant to, the Registration Rights Agreement) and will be subject to the restrictions
applicable thereto under the Registration Rights Agreement. Any transfer of any share of GX Common Stock in violation of any provision
of this Sponsor Agreement or the Registration Rights Agreement shall be void ab initio and of no force or effect. For the avoidance of
doubt, and as set forth in the Exchange Agreement, no Earnout Share that does not constitute a Released Earnout Share will be exchangeable
pursuant to the Exchange Agreement.


(c)         
Procedures Applicable to the Earnout Shares.


(i)     
As of and following the Second Merger Effective Time, the books and records of GX or the Company, as applicable, evidencing the Earnout
Shares shall be stamped or otherwise imprinted with a legend, or if held in book entry, shall be transferred to a restricted account
that is subject to a trading restrictions consistent with a legend, in substantially the following form (collectively, the “Legend”):


THE
SECURITIES EVIDENCED HEREIN ARE SUBJECT TO RESTRICTIONS ON TRANSFER, AND CERTAIN OTHER AGREEMENTS, SET FORTH IN THE SPONSOR SUPPORT AGREEMENT,
DATED AS OF SEPTEMBER 25, 2022, BY AND




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AMONG
GX ACQUISITION CORP. II, NIOCORP DEVELOPMENTS LTD. AND THE OTHER PARTIES THERETO.


(ii)     
As soon as practicable on the date upon which any Earnout Shares become Released Earnout Shares, GX or the Company, as applicable, shall
promptly remove, or cause to be removed, the Legend from the books and records of GX or the Company, as applicable, with respect to such
Released Earnout Shares and such shares shall no longer be subject to any of the terms of this Section 2.5.


(iii)     
If any Earnout Shares have not become Released Earnout Shares on or before the tenth (10th) anniversary of the Closing Date (the period
from the Second Merger Effective Time through (and including) such tenth (10th) year anniversary of the Closing Date, the “Earnout
Share Period”), then (A) such Earnout Shares shall immediately and automatically thereafter be forfeited to GX or the Company,
as applicable, and canceled, (B) each applicable Sponsor shall cause all right, title and interest therein to be transferred to GX or
the Company, as applicable, without consideration, and (C) each applicable Sponsor shall not have any rights with respect thereto. GX
or the Company, as applicable, is authorized to deliver any notices required to be delivered to its transfer agent and take such further
actions in order to redeem or terminate, as applicable, and cancel any Earnout Shares that have been forfeited as provided in this Section
2.5(c)(iii); provided that for greater certainty, for purposes of National Instrument 62-104 – Take-Over Bids and Issuer
Bids, no valuable consideration shall be deemed offered or paid to a holder of Company Common Shares in connection with a redemption
of Company Common Shares pursuant to this Section 2.5(c)(iii).


(d)         
Release of Earnout Shares. The Earnout Shares shall become Released Earnout Shares as follows:


(i)     
1,575,000 of the Earnout Shares (provided that such number is measured as of the date of this Sponsor Agreement and will be adjusted
as provided in the definition of “Earnout Shares”) shall constitute “Released Earnout Shares” on the date following
the Second Merger Effective Time on which the VWAP of one Company Common Share has equaled or exceeded $13.42 per share divided by the
Exchange Ratio, which such value assumes no Reverse Stock Split will have occurred (the “First Tranche Release Price”)
for twenty (20) of any thirty (30) consecutive trading days during the Earnout Share Period on any stock exchange on which the Company
Common Shares are then trading (“Tranche 1 Earnout Shares”); and


(ii)     
1,575,000 of the Earnout Shares (provided that such number is measured as of the date of this Sponsor Agreement and will be adjusted
as provided in the definition of “Earnout Shares”) shall constitute “Released Earnout Shares” on the date following
the Second Merger Effective Time on which the VWAP of one Company Common Share has equaled or exceeded $16.77 per share divided by the
Exchange Ratio, which such value assumes no Reverse Stock Split will have occurred (the “Second Tranche Release Price”)
for twenty (20) of any thirty (30) consecutive trading days during the




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Earnout
Share Period on any stock exchange on which the Company Common Shares are then trading (“Tranche 2 Earnout Shares”).


(e)         
Equitable Adjustments. The First Tranche Release Price and Second Tranche Release Price shall be equitably adjusted for any recapitalization,
reclassification, consolidation, subdivision, combination, amalgamation, arrangement, exchange of shares or other similar event with
respect to the Company Common Shares after the date of this Sponsor Agreement (including, for the avoidance of doubt, the actual Reverse
Stock Split).


(f)          
Acceleration Event. If, during the period beginning at the Closing and ending on the tenth (10th) anniversary of the Second Merger
Effective Time, there is a Change in Control, then any Earnout Shares shall be deemed to constitute Released Earnout Shares immediately
prior to the consummation of such Change in Control.


(g)         
Dividends. From and after the Closing, the Company hereby agrees not to declare or pay any dividends or other distributions to
its shareholders unless GX pays, and the Company causes GX to pay, to the Sponsors their portion thereof with respect to their Vested
Shares and Earnout Shares then outstanding which have not been exchanged pursuant to the Exchange Agreement, on an as-exchanged (pursuant
to the Exchange Agreement) to Company Common Share basis, of such dividends or other distributions, pursuant to and in accordance with
the Certificate of Incorporation.


Section
2.6            Advisor Shares. Prior to the date hereof, GX and the Company
have entered into certain agreements with Cantor Fitzgerald & Co. or BTIG LLC in their roles as advisors to GX in the Transactions
(each, an “Advisor”), in each case set forth on Section 4.14 of the GX Disclosure Letter to the Business Combination
Agreement, agreeing that a portion of such Advisor’s fees will, contingent upon the Closing occurring, be settled in (a) Company
Common Shares issued to an Advisor at Closing (the “Advisor Company Common Shares”) or (b) cash paid to such Advisor
to be used for the purchase of Class A Shares prior to the First Merger Effective Time (the “Advisor Class A Shares”).
In connection with the entry into such agreements, each Sponsor agrees to forfeit, at the Closing, its pro rata share of a number of
shares of GX Common Stock equal, in the aggregate when taken together with the shares of GX Common Stock forfeited by all other Sponsors,
to 50% of the Advisor Company Common Shares (collectively, the “Forfeited Shares”), as follows:


(a)         
50% of the Forfeited Shares shall be Vested Shares.


(b)         
25% of the Forfeited Shares shall be Tranche 1 Earnout Shares.


(c)         
25% of the Forfeited Shares shall be Tranche 2 Earnout Shares.


All
Advisor Company Common Shares, once Transferred to the applicable Advisor, and all Advisor Class A Shares, once issued to the applicable
Advisor, shall be fully vested and freely transferrable and not subject to any price-based vesting or other transfer restrictions applicable
to Earnout Shares, in each case subject to applicable Law.




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Section
2.7           Anti-Dilution Waiver. Each Sponsor
waives, to the fullest extent permitted by Law, the ability to adjust the Initial Conversion Ratio (as defined in the Certificate of
Incorporation) pursuant to Section 4.3(b) of the Certificate of Incorporation in connection with the issuance of additional Class A Shares
in the Transactions. This waiver shall be applicable only in connection with the Transactions and this Sponsor Agreement (and any Class
A Shares issued in connection with the Transactions) and shall be void and of no force and effect following the Support Expiration Time.


Section
2.8           Further Assurances. Each Sponsor shall take, or cause to be taken, all actions and do, or cause to be done, all things reasonably
necessary under applicable Laws to consummate the Transactions on the terms and subject to the conditions set forth therein and herein.
Without limiting the foregoing, until the Support Expiration Time, no later than five Business Days prior to any meeting of the shareholders
of GX, each Sponsor shall deliver or cause to be delivered to GX duly executed proxies or voting instruction forms voted in accordance
with the terms hereof, such proxy or voting instruction forms not to be revoked or withdrawn without the prior written consent of the
Company. No Sponsor will communicate with any shareholder of GX in a manner inconsistent with the GX Board recommendation in favor of
the Transactions and the Business Combination Agreement.


Section
2.9           No Inconsistent Agreement. Each Sponsor represents and covenants that such Sponsor has not entered into, and shall not enter into,
any agreement, or amend or modify any existing agreement, that would restrict, limit or interfere with the performance of such Sponsor’s
obligations hereunder.


Section
2.10        Binding Effect of Business Combination Agreement. Each Sponsor shall be bound by
and comply with Sections 5.2(h) and 5.8 of the Business Combination Agreement as if (a) such Sponsor was an original signatory to the
Business Combination Agreement with respect to such provisions and (b) each reference to “GX” in Sections 5.2(h) and 5.8
of the Business Combination Agreement also referred to each Sponsor.


Section
2.11         Tax Treatment. Each party hereto agrees to report the Transactions in all cases in accordance with the Intended Tax Treatment
except, and then solely to the extent, otherwise required by a final determination under applicable Law.


ARTICLE
III

REPRESENTATIONS AND WARRANTIES


Section
3.1           Representations and Warranties of the Sponsors. Each Sponsor represents and warrants as of the date hereof to GX and the Company
(solely with respect to itself, himself or herself and not with respect to any other Sponsor) as follows:


(a)         
Organization; Due Authorization. If such Sponsor is not an individual, it is duly organized, validly existing and in good standing
under the Laws of the jurisdiction in which it is incorporated, formed, organized or constituted, and the execution, delivery and performance
of this Sponsor Agreement and the consummation of the transactions contemplated hereby are within such Sponsor’s corporate, limited
liability company or




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organizational
powers and have been duly authorized by all necessary corporate, limited liability company or organizational actions on the part of such
Sponsor. If such Sponsor is an individual, such Sponsor has full legal capacity, right and authority to execute and deliver this Sponsor
Agreement and to perform his or her obligations hereunder. This Sponsor Agreement has been duly executed and delivered by such Sponsor
and, assuming due authorization, execution and delivery by the other parties to this Sponsor Agreement, this Sponsor Agreement constitutes
a legally valid and binding obligation of such Sponsor, enforceable against such Sponsor in accordance with the terms hereof (subject
to the Enforceability Exceptions). If this Sponsor Agreement is being executed in a representative or fiduciary capacity, the Person
signing this Sponsor Agreement has full power and authority to enter into this Sponsor Agreement on behalf of the applicable Sponsor.


(b)              
Ownership. Such Sponsor is the record and beneficial owner (as defined in the Securities Act) of, and has good title to, all of
such Sponsor’s shares of GX Common Stock and GX Warrants, and there exist no Liens or any other limitation or restriction (including
any restriction on the right to vote, sell or otherwise dispose of such shares of GX Common Stock or GX Warrants (other than transfer
restrictions under the Securities Act)) or preemptive or other right or privilege for the purchase, acquisition or transfer from such
Sponsor affecting any such shares of GX Common Stock or GX Warrants, other than Liens pursuant to (i) this Sponsor Agreement, (ii) the
GX organizational documents, (iii) the Business Combination Agreement, (iv) the Letter Agreement, dated March 17, 2021, by and among
the Sponsors and GX (the “Voting Letter Agreement”), or (v) any applicable securities Laws. As of the date of this
Sponsor Agreement, such Sponsor’s shares of GX Common Stock and GX Warrants are the only equity securities in GX owned of record
or beneficially by such Sponsor or over which such Sponsor has voting control or direction, and none of such Sponsor’s shares of
GX Common Stock or GX Warrants are subject to any proxy, voting trust or other agreement or arrangement with respect to the voting of
such shares of GX Common Stock or GX Warrants, except as provided hereunder and under the Voting Letter Agreement. Other than the GX
Warrants, such Sponsor does not hold or own any rights to acquire (directly or indirectly) any equity securities of GX or any equity
securities convertible into, or which can be exchanged for, equity securities of GX.


(c)              
No Conflicts. The execution and delivery of this Sponsor Agreement by such Sponsor does not, and the performance by such Sponsor
of his, her or its obligations hereunder will not, (i) if such Sponsor is not an individual, conflict with or result in a violation of
the organizational documents of such Sponsor or (ii) require any consent or approval that has not been given or other action that has
not been taken by any Person (including under any contract binding upon such Sponsor or such Sponsor’s shares of GX Common Stock
or GX Warrants), in each case, to the extent such consent, approval or other action would prevent, enjoin or materially delay the performance
by such Sponsor of its, his or her obligations under this Sponsor Agreement.


(d)              
Litigation. There are no Actions pending against such Sponsor, or to the knowledge of such Sponsor threatened against such Sponsor,
before (or, in the case of threatened Actions, that would be before) any arbitrator or any Governmental Entity, which in any manner challenges
or seeks to prevent, enjoin or materially delay the performance by such Sponsor of its, his or her obligations under this Sponsor Agreement.




    	 	10	 



    	 



    

(e)              
 Brokerage Fees. Except as set forth on Section 4.14 of the GX Disclosure Letter, no broker, finder, investment banker or other
Person is entitled to any brokerage fee, finders’ fee or other commission in connection with the Transactions upon arrangements
made by such Sponsor, for which GX or any of its Affiliates may become liable.


(f)               
Acknowledgment. Such Sponsor understands and acknowledges that each of GX and the Company is entering into the Business Combination
Agreement in reliance upon such Sponsor’s execution and delivery of this Sponsor Agreement.


(g)              
Disclosure. Each of the undersigned hereby consents to the disclosure of this Sponsor Agreement and the terms and conditions thereof
in any press release or other documents filed with pursuant to applicable securities laws in connection with the Business Combination
Agreement or the Transactions.


ARTICLE
IV

MISCELLANEOUS


Section
4.1           
Termination. This Sponsor Agreement and all of its provisions shall terminate and be of no further force or effect upon the earliest
of (a) solely with respect to Sections 2.1 through 2.4 and Section 2.6, the Support Expiration Time; provided, however,
that the other provisions of this Sponsor Agreement (including Section 2.5) shall not terminate upon, and shall continue to be
in full force and effect in accordance with their terms, following the Support Expiration Time if, and only if, the Transactions have
closed, (b) the liquidation of the GX trust account upon the failure of GX to consummate its initial business combination within the
time period set forth in the Certificate of Incorporation, and (c) the written agreement of Sponsor Holdco, GX and the Company. Upon
a termination of this Sponsor Agreement or certain provisions hereof, as applicable, all obligations of the parties under this Sponsor
Agreement or such provisions, as applicable, will terminate, without any liability or other obligation on the part of any party hereto
to any Person in respect hereof or the transactions contemplated hereby, and no party hereto shall have any claim against another (and
no Person shall have any rights against such party), whether under contract, tort or otherwise, with respect to the subject matter hereof;
provided, however, that the termination of this Sponsor Agreement or any provision hereof shall not relieve any party hereto from liability
arising in respect of any breach of this Sponsor Agreement prior to such termination. This ARTICLE IV shall survive the termination
of this Sponsor Agreement.


Section
4.2            
Governing Law; Venue; Waiver of Jury Trial.


(a)         
This Sponsor Agreement will be construed and enforced in accordance with the Laws of the State of Delaware, without regard to the conflict
of laws principles that would result in the application of any Law other than the Law of the State of Delaware.


(b)         
Each of the parties hereby irrevocably and unconditionally submits, for itself and its property, to the exclusive jurisdiction and venue
of the Court of Chancery of the State of Delaware (or, to the extent that such Court does not have subject matter jurisdiction, the Superior
Court of the State of Delaware) or, if it has or can acquire jurisdiction, in the United




    	 	11	 



    	 



    

States
District Court for the District of Delaware (collectively, the “Chosen Courts”), in any proceeding arising out of
or relating to this Sponsor Agreement or the agreements delivered in connection herewith or the Transactions or for recognition or enforcement
of any judgment relating thereto, and each of the parties hereby irrevocably and unconditionally (i) agrees not to commence any
such proceeding, except in the Chosen Courts; (ii) agrees that any claim in respect of any such proceeding may be heard and determined
in the Chosen Courts; (iii) waives, to the fullest extent it may legally and effectively do so, any objection that it may now or
hereafter have to the laying of venue of any such proceeding in the Chosen Courts; and (iv) waives, to the fullest extent permitted
by Law, the defense of an inconvenient forum to the maintenance of such proceeding in the Chosen Courts. Each of the parties agrees that
a final judgment in any such proceeding shall be conclusive and may be enforced in other jurisdictions by suit on the judgment or in
any other manner provided by applicable Law. Each party irrevocably consents to service of process inside or outside the territorial
jurisdiction of the Chosen Courts the manner provided for notices in Section 4.7. Nothing in this Sponsor Agreement will
affect the right of any party to this Sponsor Agreement to serve process in any other manner permitted by applicable Law.


(c)         
EACH PARTY HEREBY IRREVOCABLY WAIVES, AND WILL CAUSE ITS SUBSIDIARIES AND AFFILIATES TO WAIVE, ANY AND ALL RIGHTS TO TRIAL BY JURY IN
ANY LEGAL PROCEEDING ARISING OUT OF OR RELATED TO THIS SPONSOR AGREEMENT OR THE TRANSACTIONS.


Section
4.3            Assignment. This Sponsor Agreement will be binding upon
and inure to the benefit of the parties and their respective successors, legal representatives and permitted assigns. No party may assign
any of its rights or delegate any of its obligations under this Sponsor Agreement, by operation of Law or otherwise, without the prior
written consent of the other Party.


Section
4.4            Specific Performance. The parties agree that irreparable damage for which monetary damages, even if available, would not be an
adequate remedy, would occur in the event that the parties do not perform their obligations under the provisions of this Sponsor Agreement
(including failing to take such actions as are required of them hereunder to consummate this Sponsor Agreement) in accordance with its
specified terms or otherwise breach such provisions. Subject to the other provisions of this Section 4.4, the parties acknowledge
and agree (and further agree not to take any contrary position in any litigation concerning this Sponsor Agreement) that (a) the
parties will be entitled to an injunction or injunctions, specific performance, or other equitable relief, to prevent breaches of this
Sponsor Agreement and to enforce specifically the terms and provisions hereof without proof of damages or otherwise, and that such relief
may be sought in addition to and will not limit, diminish or otherwise impair, any other remedy to which they are entitled under this
Sponsor Agreement, (b) the provisions set forth herein are not intended to and do not adequately compensate for the harm that would
result from a breach of this Sponsor Agreement and will not be construed to limit, diminish or otherwise impair in any respect any party’s
right to specific enforcement, and (c) the right of specific enforcement is an integral part of the Transactions and without that
right, none of the parties would have entered into this Sponsor Agreement. The parties acknowledge and agree that any party seeking an
injunction or injunctions to prevent breaches of this Sponsor Agreement and to enforce specifically




    	 	12	 



    	 



    

the
terms and provisions of this Sponsor Agreement in accordance with this Section 4.4 will not be required to provide any bond
or other security in connection with any such order or injunction.


Section
4.5            Amendment. This Sponsor Agreement may not be amended, changed, supplemented, waived or otherwise modified or terminated, except
upon the execution and delivery of a written agreement executed by GX, the Company and the Sponsor Holdco.


Section
4.6            Severability. The provisions of this Sponsor Agreement will be deemed severable and the invalidity or unenforceability of any
provision will not affect the validity or enforceability of the other provisions hereof. If any provision of this Sponsor Agreement,
or the application thereof to any Person or any circumstance, is invalid or unenforceable, then (a) a suitable and equitable provision
will be substituted therefor in order to carry out, so far as may be valid and enforceable, the intent and purpose of such invalid or
unenforceable provision and (b) the remainder of this Sponsor Agreement and the application of such provision to other Persons or
circumstances will not be affected by such invalidity or unenforceability, nor will such invalidity or unenforceability affect the validity
or enforceability of such provision, or the application thereof, in any other jurisdiction.


Section
4.7            Notices. All notices and other communications among the parties will be in writing and will be deemed to have been duly given
(a) when delivered in person, (b) when delivered by FedEx or other nationally recognized overnight delivery service, or (c) when
delivered by email (so long as the sender of any such e-mail has not received an e-mail from the applicable server indicating a delivery
failure), in each case, according to the instructions set forth below. Such notices will be deemed given: at the time of personal delivery,
if delivered in person; one (1) Business Day after being sent, if sent by reputable, overnight delivery service and at the time sent
(so long as the sender of any such e-mail has not received an e-mail from the applicable server indicating a delivery failure), if sent
by email prior to 5:00 p.m. local time of the recipient on a Business Day; or on the next Business Day if sent by email after 5:00 p.m.
local time of the recipient on a Business Day or on a non-Business Day.


If
to GX:


GX
Acquisition Corp. II

1325 Avenue of the Americas, 28th Floor

New York, New York 10019








	 	Attention:
	Jay Bloom
	 	 	Dean
Kehler
	 	Email:	jay.bloom@trimarancapital.com
	 	 	dean.kehler@trimarancapital.com



 


with
a copy to (which will not constitute notice):




    	 	13	 



    	 



    

Skadden,
Arps, Slate, Meagher & Flom LLP

One Manhattan West

New York, New York 10001


	 	Attention:	Michael Chitwood
	 	 	Michael Civale
	 	Email:	Michael.Chitwood@skadden.com
	 	 	Michael.Civale@skadden.com






If
to a Sponsor: 





To
such Sponsor’s address set forth in Schedule I

with a copy to (which will not constitute notice):





Skadden,
Arps, Slate, Meagher & Flom LLP

One Manhattan West

New York, New York 10001








	 	Attention:	Michael Chitwood
	 	 	Michael Civale
	 	Email:	Michael.Chitwood@skadden.com
	 	 	Michael.Civale@skadden.com





If
to the Company:





NioCorp
Developments Ltd.
 7000 South Yosemite Street, Suite 115
 Centennial, CO 80112


	 	Attention:	 Mark Smith
	 	 	Neal Shah
	 	Email:	msmith@niocorp.com
	 	 	nshah@niocorp.com






with
a copy to (which shall not constitute notice):


 


Jones
Day
 250 Vesey Street
 New York, NY 10281








	 	Attention:
	Joel May
	 	 	Ann Bomberger
	 	Email:	jtmay@jonesday.com
	 	 	ambomberger@jonesday.com






Section
4.8          Counterparts. This Sponsor Agreement may be executed in any number of counterparts (including by pdf or other readable electronic
format), each such counterpart being deemed to be an original instrument, with the same effect as if the signature thereto and hereto
were upon the same instrument, and will become effective when one or more counterparts have been signed by each of the parties and delivered
(including by email or




    	 	14	 



    	 



    

DocuSign)
to the other parties, and all such counterparts will together constitute one and the same agreement.


Section
4.9          Entire Agreement. This Sponsor Agreement and the agreements referenced herein constitute the entire agreement, and supersede all
other prior agreements, understandings, representations and warranties both written and oral, among the Parties, with respect to the
subject matter hereof.


Section
4.10         No Third-Party Beneficiaries. The representations,
warranties and covenants set forth herein are solely for the benefit of the other Parties, in accordance with and subject to the terms
of this Sponsor Agreement, and this Sponsor Agreement is not intended to, and does not, confer
upon any Person other than the Parties any rights or remedies hereunder, including the right to rely upon the representations and warranties
set forth herein.


[THE
REMAINDER OF THIS PAGE IS INTENTIONALLY BLANK]




    	 	15	 



    	 



    

IN
WITNESS WHEREOF, the Sponsors, GX and the Company have each caused this Sponsor Support Agreement to be duly executed as of the date
first written above.


 


 


	 	SPONSORS:
	 	 
	 	GX SPONSOR II LLC
	 	 
	 	 
	 	 
	 	 By:  	/s/ Dean Kehler
	 	 	Name:  Dean Kehler  
	 	 	Title:   Managing Member
	 	 	 	 
	 	 	 	 
	 	 By:  	Cooper Road, LLC, Managing Director
	 	 	 	 
	 	 	 	 
	 	 By:  	/s/ Jay R. Bloom 
	 	 	Name:  Jay R. Bloom 
	 	 	Title:   Managing Member
	 	 	 	 
	 	 	 	 
	 	 	 	 
	 	/s/ Jay R. Bloom
	 	Name: Jay R. Bloom
	 	 
	 	/s/ Dean C. Kehler
	 	Name: Dean C. Kehler
	 	 
	 	/s/ Michael G. Maselli
	 	Name: Michael G. Maselli
	 	 
	 	/s/ Andrea J. Kellet
	 	Name:  Andrea J. Kellet
	 	 
	 	/s/ Hillel Weinberger
	 	Name:  Hillel Weinberger
	 	 
	 	/s/ James W. Harpel
	 	Name: James W. Harpel







 


 




    	 	[Signature Page to Sponsor Support Agreement]	 



    	 



    

 





 


	 	/s/ Arthur D. Baer
	 	Name:  Arthur D. Baer
	 	 
	 	/s/ Jordan S. Bloom
	 	Name:  Jordan S. Bloom
	 	 
	 	GX:
	 	 
	 	 
	 	GX ACQUISITION CORP. II
	 	 
	 	 
	 	 By:  	/s/ Dean Kehler 
	 	 	Name:  Dean Kehler  
	 	 	Title:    Co-Chairman and Chief Executive Officer
	 	 	 	 




 




    	 	[Signature Page to Sponsor Support Agreement]	 



    	 



    

 





 


		COMPANY:
	 	 
	 	NIOCORP DEVELOPMENTS LTD.
	 	 
	 	 
	 	 By:  	/s/ Mark Smith 
	 	 	Name:  Mark Smith  
	 	 	Title:    Chief Executive Officer
	 	 	 	 




  


 




    	 	[Signature Page to Sponsor Support Agreement]	 



    	 



    

Schedule
I


Sponsor
GX Common Stock, GX Warrants, Vested Shares and Earnout Shares 


 


	Sponsor
    	GX
    Class B Common Shares	GX
    Warrants	Vested
    Shares	Earnout
    Shares
	GX
Sponsor LLC

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	7,500,000	5,666,667	4,350,000
    (2)(3)	3,150,000(4)
	Jay
R. Bloom

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	7,500,000
    (1)	5,666,667
    (1)	4,350,000
    (1)(3)	3,150,000
    (1)(4)
	Dean
C. Kehler

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	7,500,000
    (1)	5,666,667
    (1)	4,350,000
    (1)(3)	3,150,000
    (1)(4)
	Michael
G. Maselli

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	—
    (2)	—
    (2)	—
    (2)	—
    (2)
	Andrea
J. Kellett

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	—
    (2)	—
    (2)	—
    (2)	—
    (2)
	Hillel
Weinberger

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	—
    (2)	—
    (2)	—
    (2)	—
    (2)
	Marc
Mazur

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	—
    (2)	—
    (2)	—
    (2)	—
    (2)
	James
W. Harpel

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	—
    (2)	—
    (2)	—
    (2)	—
    (2)
	Arthur
D. Baer

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	—
    (2)	—
    (2)	—
    (2)	—
    (2)
	Jordan
S. Bloom

c/o GX Acquisition Corp.

1325 Avenue of the Americas, 28th Floor, New York, NY 10019
	—
    (2)	—
    (2)	—
    (2)	—
    (2)


 


(1)       Cooper
Road, LLC, an entity controlled by Jay R. Bloom, and Dean C. Kehler, is the managing member of GX Sponsor II LLC, and as such Messrs.
Bloom and Kehler have voting and investment discretion with respect to the GX Common Stock and GX Warrants held of record by GX Sponsor
LLC and may be deemed to have shared beneficial ownership of the GX Common Stock and GX Warrants held directly by GX Sponsor LLC. Each
such entity or person disclaims any beneficial ownership of such shares and warrants other than to the extent of any pecuniary interest
they may have therein, directly or indirectly.




    	 	[Schedule I to Sponsor Support Agreement]	 



    	 



    

(2)       Each
of these individuals holds an interest in GX Sponsor II LLC. Each such person disclaims any beneficial ownership of the reported shares
other than to the extent of any pecuniary interest they may have therein, directly or indirectly.


 (3)       48,645,706
Second Merger Surviving Company Shares shall be issued with respect to such Vested Shares in the Second Merger.


 (4)       35,226,201
Second Merger Surviving Company Shares shall be issued with respect to such Earnout Shares in the Second Merger.


 






    	 	[Schedule I to Sponsor Support Agreement]