<PAGE>
                                                                     Exhibit 4.2

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                                G-I HOLDINGS INC.

                                       AND

                              THE BANK OF NEW YORK,


                                     Trustee

                                ----------------

                          FIRST SUPPLEMENTAL INDENTURE

                          Dated as of October 18, 1996

                                       TO

                                    INDENTURE

                           Dated as of October 5, 1993

                                             
                                ----------------



                              Senior Discount Notes

                               Due October 1, 1998

                                       and

                         Series B Senior Discount Notes

                               Due October 1, 1998


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<PAGE>
     

               FIRST SUPPLEMENTAL INDENTURE, dated as of October 18, 1996,
     between G-I HOLDINGS INC., a Delaware corporation (the "Company"), and
     THE BANK OF NEW YORK, a New York banking corporation (the "Trustee"),
     having its Corporate Trust Office at 101 Barclay Street, New York, New
     York 10286.


                                    RECITALS

               WHEREAS, the Company and the Trustee have executed and
     delivered the Indenture, dated as of October 5, 1993 (the "Original
     Indenture" and capitalized terms used herein without definition have
     the respective meanings specified therein), governing the terms of the
     Company's Senior Discount Notes due 1998 and Series B Senior Discount
     Notes due 1998 (collectively, the "Notes"); and  

               WHEREAS, ISP Holdings Inc. ("ISP Holdings") has solicited
     the consent of the holders of the Notes to certain amendments (the
     "Amendments") to the Original Indenture pursuant to that certain Offer
     to Purchase and Consent Solicitation Statement of ISP Holdings, dated
     September 13, 1996, as amended (the "Offer to Purchase"); and

               WHEREAS, Holders representing a majority in aggregate
     principal amount of the Notes have delivered their consent to the
     Amendments; and

               WHEREAS, Section 9.02 of the Original Indenture permits the
     Company, when authorized by resolution of its Board of Directors, and
     the Trustee, to amend the Original Indenture with the written consent
     of the Holders of a majority in aggregate principal amount of the
     Notes then outstanding; and

               WHEREAS, the Board of Directors of the Company has adopted
     such a resolution in order to reflect the Amendments pursuant to this
     First Supplemental Indenture; and

               WHEREAS, the Company desires to enter into this First
     Supplemental Indenture in order to amend the Original Indenture as of
     the Effective Time (as defined herein);

               NOW, THEREFORE, in consideration of the premises and for
     other good and valuable consideration, it is mutually covenanted and
     agreed for the equal and ratable benefit of all Holders of the Notes
     as follows:




<PAGE>
     

                                   ARTICLE ONE
     
                        AMENDMENTS TO ORIGINAL INDENTURE


          At the Effective Time, each of the following sections of the
     Original Indenture shall be modified as follows:


          A.   Definitions.

               -----------
               (i)  Section 1.01 shall be amended by adding the following
     definitions in their appropriate alphabetical location:

               "ISP Holdings Transactions" means the consummation of a 
                -------------------------
               tender offer, exchange offer and senior note offering of ISP
               Holdings Inc., on substantially the terms described in that
               certain Offer to Purchase and Consent Solicitation
               Statement, dated September 13, 1996, as amended, of ISP
               Holdings Inc.

               "Spin Off Transactions" means the consummation of a series 
                ---------------------
               of transactions involving GAF's subsidiaries on
               substantially the terms described in that certain Offer to
               Purchase and Consent Solicitation Statement, dated September
               13, 1996, as amended, of ISP Holdings Inc. that will, among
               other things, result in the capital stock of ISP Holdings
               Inc. being distributed to the stockholders of GAF.

               (ii)  Section 1.01 shall be amended further by deleting each
     of the following definitions in its entirety:

               "Linden Dividend"
                ---------------
               "Linden Property"
                ---------------
               "Permitted Lien"
                --------------
               "Restricted Investment"
                ---------------------
               "Restricted Payment"
                ------------------



<PAGE>
     

               "Tax Sharing Agreements"
                ----------------------

               "Unrestricted Affiliate"
                ----------------------

          B.   Covenants.
               ---------
               (i)  The text of each of the following Sections shall be
     deleted in its entirety and replaced, in each case, by the words
     "Intentionally Omitted":

               Section 4.12.  Limitation on Restricted Payments and
                              -------------------------------------

     Restricted Investments.
     ----------------------
               Section 4.13.  Limitation on Liens.
                              -------------------
               Section 4.14.  Limitation on Transactions with Affiliates.
                              ------------------------------------------
               Section 4.15.  Limitation on Investments in Non-Recourse
                              -----------------------------------------
     Subsidiaries by ISP Subsidiaries and BMC Subsidiaries.
     -----------------------------------------------------
               Section 4.16.  Limitation on Dividend and Other Payment
                              ----------------------------------------
     Restrictions Affecting Subsidiaries.
     -----------------------------------
               Section 4.20.  Consents, etc.
                              --------------
               (ii)  Each of the following Sections shall be amended to
     read in its entirety as follows:

               Section 4.04.  Payments of Taxes and Other Claims.  G-I
                              ----------------------------------
     Holdings shall, and shall cause each of its Subsidiaries (other than
     Non-Recourse Subsidiaries) to, pay or discharge or cause to be paid or
     discharged, before any penalty accrues from the failure to so pay or
     discharge, all material taxes, assessments and governmental charges
     levied or imposed upon it or any of such Subsidiaries or upon the
     income, profits or property of it or any of such Subsidiaries,
     provided that there shall not be required to be paid or discharged any
     --------
     such tax, assessment or charge if the amount, applicability or
     validity thereof is being contested in good faith by appropriate
     proceedings and adequate provision therefor has been made.

               Section 5.01.  When G-I Holdings May Merge, etc.  G-I
                              --------------------------------
     Holdings shall not consolidate with or merge with or into or sell,
     assign, transfer or lease all or substantially all of its properties
     and assets (either in one transaction or series of


<PAGE>
     

     related transactions) to any Person, unless G-I Holdings shall be the
     continuing Person, or the resulting, surviving or transferee Person
     (if other than G-I Holdings) shall be a corporation organized and
     existing under the laws of the United States or any State thereof or
     the District of Columbia and shall expressly assume, by an indenture
     supplemental hereto, executed and delivered to the Trustee, in form
     reasonably satisfactory to the Trustee, all the obligations of G-I
     Holdings under the Securities and this Indenture, and this Indenture
     shall remain in full force and effect.

               In connection with any consolidation, merger, sale,

     assignment, transfer or lease contemplated by this Section 5.01, G-I
     Holdings shall deliver, or cause to be delivered, to the Trustee, in
     form and substance reasonably satisfactory to the Trustee, an
     Officers' Certificate and an Opinion of Counsel, each stating that
     such consolidation, merger, sale, assignment, transfer or lease and
     the supplemental indenture in respect thereto comply with this
     Article V and that all conditions precedent herein provided for
     relating to such transaction have been complied with.

               Section 6.01.  Events of Default.  An "Event of Default"
                              -----------------
     occurs if:

                         (1)  G-I Holdings defaults in the payment of
                    interest on any Security when the same becomes due and
                    payable and the default continues for a period of 30
                    days;

                         (2)(i)  G-I Holdings defaults in the payment of
                    the principal of any Security when the same becomes due
                    and payable at maturity or otherwise or (ii) G-I
                    Holdings fails to redeem or repurchase Securities when
                    required pursuant to this Indenture or the Securities;

                         (3)  G-I Holdings fails to comply with Section
                    5.01;

                         (4)  G-I Holdings fails to comply for 30 days
                    after notice with any of its obligations under
                    Sections 4.03, 4.06, 4.09, 4.10, 4.11 and 4.17;

                         (5)  G-I Holdings fails to comply for 60 days
                    after notice with its other agreements contained in
                    this Indenture or the Securities (other than those
                    referred to in clauses (1)-(4) above); or


<PAGE>
     

                         (6)  G-I Holdings or any of its Significant
                    Subsidiaries (A) admits in writing its inability to pay
                    its debts generally as they become due, (B) commences a
                    voluntary case or proceeding under any Bankruptcy Law
                    with respect to itself, (C) consents to the entry of a
                    judgment, decree or order for relief against it in an
                    involuntary case or proceeding under any Bankruptcy
                    Law, (D) consents to the appointment of a Custodian of
                    it or for substantially all of its property, (E)
                    consents to or acquiesces in the institution of a
                    bankruptcy or an insolvency proceeding against it, (F)
                    makes a general assignment for the benefit of its
                    creditors, or (G) takes any corporate action to
                    authorize or effect any of the foregoing.


               A Default under clauses (4) or (5) is not an Event of
     Default until the Trustee or the Holders of at least 25% in aggregate
     principal amount of the outstanding Securities notify G-I Holdings in
     writing of the Default, and G-I Holdings does not cure the Default
     with the time specified in such clause after receipt of such notice. 
     Such notice shall be given by the Trustee if so requested in writing
     by the Holders of at least 25% in aggregate principal amount of the
     outstanding Securities.  When a Default under clause (4) or (5) is
     cured or remedied within the specified period, it ceases to exist.

               (iii)  Each of Sections 4.09, 4.10 and 4.11 shall be amended
     by modifying the ratio set forth in subparagraph (a) of each such
     Section to read "at least 1.00 to 1.00."  Section 4.09 shall further
     be amended by deleting subparagraph (e) therein in its entirety.  Each
     of Sections 6.02 and 7.07 shall be amended by changing each reference
     therein to clause (7) of Section 6.01 to be a reference to clause (6)
     of Section 6.01.

               (iv)  Subparagraph (a) of Section 4.06 shall be amended to
     read in its entirety as follows:

               (a)  G-I Holdings will file with the Trustee and provide
     Securityholders, within 15 days after it files them with the
     Commission (and only to the extent that it files them with the
     Commission), copies of its annual report and the information,
     documents and other reports (or copies of such portions of any of the
     foregoing as the Commission may by rules and regulations prescribe)
     which the Company is required to file with the Commission pursuant to
     Section 13 of 15(d) of the Exchange Act, without exhibits in the case
     of Securityholders, unless G-I



<PAGE>
     

     Holdings is requested in writing by the Securityholders.  G-I Holdings
     also will comply with the TIA Section 314(a).

               (v)  Subparagraph (a) of Section 4.18 shall be amended to
     read in its entirety as follows:

               (a)  G-I Holdings shall not, and shall not permit any of its
     Subsidiaries, directly or indirectly, to consummate an Asset Sale
     unless:

               (1)  in the case of an Asset Sale by G-I Holdings or any
          Specified Subsidiary, G-I Holdings shall commit to apply the Net
          Cash Proceeds of such Asset Sale within 300 days of the
          consummation of such Asset Sale, and shall apply such Net Cash
          Proceeds within 360 days of receipt thereof, (i) to invest in the
          businesses that G-I Holdings and its Subsidiaries (other than
          businesses engaged in through Non-Recourse Subsidiaries) are

          engaged in at the time of such Asset Sale or any like or related
          business, (ii) to pay the Debt referred to in the last sentence
          of the definition thereof or make provision for the payment
          thereof, through an escrow or other fund, and/or (iii) to offer
          to purchase the Securities in a tender offer (a "Net Proceeds
          Offer") at a redemption price equal to 100% of the Accreted Value
          thereof; provided that G-I Holdings may defer making a Net 
                   --------
          Proceeds Offer until the aggregate Net Cash Proceeds from Asset
          Sales to be applied pursuant to this clause (1)(iii) equal or
          exceed $20,000,000; and

               (2)  in the case of an Asset Sale by any ISP Subsidiary or
          any BMC Subsidiary, such ISP Subsidiary or BMC Subsidiary, as the
          case may be, shall apply the Net Cash Proceeds of such Asset Sale
          within one year of receipt thereof, (i) to invest in the
          businesses that G-I Holdings and its Subsidiaries (other than
          businesses engaged in through Non-Recourse Subsidiaries) are
          engaged in at the time of such Asset Sale or any like or related
          business, (ii) to pay the Debt referred to in the last sentence
          of the definition thereof or make provision for the payment
          thereof, through an escrow or other fund, (iii) to pay or satisfy
          Debt or Preferred Stock of any ISP Subsidiary or any BMC
          Subsidiary, as the case may be, and/or (iv) to make a Net
          Proceeds Offer at a redemption price equal to 100% of the
          Accreted Value thereof; provided that G-I Holdings may defer 
                                  --------
          making a Net Proceeds Offer until the aggregate Net Cash Proceeds
          from Asset Sales to be applied pursuant to clause (2)(iv) equal
          or exceed $20,000,000; 



<PAGE>
     

     provided that (i) G-I Holdings and its Subsidiaries may retain up to
     --------
     $5,000,000 of Net Cash Proceeds from Asset Sales in any twelve-month
     period (without complying with clauses (1) or (2)), and (ii) any Asset
     Sale that would result in a Change of Control shall not be governed by
     this Section 4.18 but shall be governed by the provisions described
     under Section 4.17 and paragraph 5(a) of the Securities.

               (vi) Subparagraph (b) of Section 8.01 shall be amended to
     read in its entirety as follows:

               (b)  Subject to Sections 8.01(c), 8.02 and 8.06, G-I
     Holdings may at any time terminate (i) all its obligations under the
     Securities and this Indenture ("legal defeasance"), or (ii) its
     obligations under Sections 4.03, 4.04, 4.06, 4.08 through 4.11 and
     4.17 through 4.19 and the operation of Section 6.01(3), 6.01(4),
     6.01(5) and 6.01(6) (with respect only to Significant Subsidiaries)
     ("covenant defeasance").


               (vii)  The penultimate paragraph of Section 8.02 shall be
     amended to read in its entirety as follows:

               Notwithstanding the foregoing provisions of this Section,
     the conditions set forth in the foregoing paragraphs (2), (3), (4),
     (5), (6), and (7) need not be satisfied so long as, at the time G-I
     Holdings makes the deposit described in paragraph (1), (i) no Default
     under Section 6.01(1), 6.01(2) or 6.01(6) has occurred and is
     continuing on the date of such deposit and after giving effect thereto
     and (ii) either (x) a notice of redemption has been mailed pursuant to
     Section 3.03 providing for redemption of all the Securities 30 days
     after such mailing and the provisions of Section 3.01 with respect to
     such redemption shall have been complied with or (y) the Stated
     Maturity of all of the Securities will occur within 30 days.  If the
     conditions of the preceding sentence are satisfied G-I Holdings shall
     be deemed to have exercised its covenant defeasance option.

               (viii)  The following new Section shall be added:

               Section 5.03.  Spin Off Transactions.  Notwithstanding any
                              ---------------------
     provision in this Indenture to the contrary, the Spin Off Transactions
     and the ISP Holdings Transactions may be consummated under all
     circumstances and without satisfying any conditions.

<PAGE>
     


                                   ARTICLE TWO

                                  MISCELLANEOUS


          A.   Governing Law.
               -------------
               The laws of the State of New York shall govern this First
     Supplemental Indenture without regard to the principles of conflict of
     laws.

          B.   Counterparts.
               ------------
               This First Supplemental Indenture may be executed in
     counterparts, each of which shall be deemed an original, but all of
     which taken together shall constitute one and the same instrument.

          C.   Survival.
               --------
               This First Supplemental Indenture and the Original Indenture
     shall henceforth be read together.  Except as expressly set forth
     herein, the Original Indenture shall remain unchanged and in full
     force and effect in accordance with its terms.


          D.   Effective Time.
               --------------
               For purposes of this First Supplemental Indenture, the
     "Effective Time" shall mean such time as is immediately prior to ISP
     Holdings purchasing, by accepting for payment, all Notes validly
     tendered (and not withdrawn) pursuant to the terms of the Offer to
     Purchase.



<PAGE>
     

               IN WITNESS WHEREOF, the parties hereto have caused this
     First Supplemental Indenture to be executed and delivered as of the
     date first above written.


     G-I HOLDINGS INC.



     By: /s/: James P. Rogers
         --------------------
        Name:  James P. Rogers
         Title: Senior Vice President



     THE BANK OF NEW YORK



     By: /s/: Walter N. Gitlin
         ---------------------
       Name:  Walter N. Gitlin
        Title: Vice President