<PAGE>
                                                                  Exhibit 10.15b


                     ADDENDUM NO. 1 TO EMPLOYMENT AGREEMENT

            This Addendum No. 1 (the "Addendum") is entered into as of February
1, 2001, between Robert E. Knapp, an individual ("Executive") and Gartner, Inc.
(formerly known as Gartner Group, Inc.), a Delaware corporation (the "Company").

                                    RECITALS

      A. Executive and the Company are parties to an Employment Agreement dated
as of August 7, 2000 (the "Employment Agreement"), which provides for Executive
to serve as Chief Marketing Officer for the Company through September 30, 2003.

      B. The Company and Executive desire to modify some of the terms of the
Employment Agreement as set forth herein in accordance with corporate guidelines
governing members of the executive leadership team of the Company as authorized
by the Board of Directors of the Company.

                                    AGREEMENT

      THEREFORE, in consideration of the mutual covenants contained herein, the
parties hereby agree as follows:

      1. Amendments. (a) Section 6(b) of the Employment Agreement is deleted in
its entirety and is replaced with the following:

            (b)   Involuntary Termination. If at any time during the term of
                  this Agreement, other than following a Change in Control to
                  which Section 6(c) applies, the Company terminates the
                  employment of Executive involuntarily and without Business
                  Reasons or a Constructive Termination occurs, then in addition
                  to salary and vacation accrued through the Termination Date,
                  Executive shall be entitled to receive the following: (i)
                  continued salary for a period of three years following the
                  Termination Date at the rate then in effect, payable in
                  accordance with the Company's regular payroll schedule as in
                  effect from time to time, (ii) at the Termination Date
                  Executive's minimum target bonus for the fiscal year in which
                  the Termination Date occurs plus any unpaid bonus from the
                  prior fiscal year, (iii) following the end of the fiscal year
                  in which the Termination Date occurs and management bonuses
                  have been determined, a pro rata share (based on the
                  proportion of the fiscal year during which Executive remained
                  an employee of the Company) of the bonus that would have been
                  payable to Executive under the bonus plan in excess of
                  Executive's minimum target bonus for the fiscal year, (iv)
                  following the end of the first fiscal year following the
                  fiscal year in which the Termination Date occurs, Executive's
                  minimum target bonus for such following fiscal year (or, if
                  the target bonus for such year was not previously set, then
                  Executive's minimum target bonus for the fiscal year in which
                  the Termination Date occurred), (v) acceleration in full of
                  vesting of all outstanding stock options, TARPS and other
                  equity arrangements subject to vesting and held by Executive
                  (and in
<PAGE>
                  this regard, all such options and other exercisable rights
                  held by Executive shall remain exercisable for one year
                  following the Termination Date, (vi) (A) for three years
                  following the Termination Date, continuation of group health
                  benefits at the Company's cost pursuant to the Company's
                  standard programs as in effect from time to time (or at the
                  Company's election substantially similar health benefits as in
                  effect at the Termination Date, through a third party carrier)
                  for Executive, his spouse and any children, and (B)
                  thereafter, to the extent COBRA shall be applicable to the
                  Company, continuation of health benefits for such persons at
                  Executive's cost, for a period of 18 months or such longer
                  period as may be applicable under the Company's policies then
                  in effect, provided the Executive makes the appropriate
                  election and payments, (vii) continuation of Executive's auto
                  benefits for one year following the Termination Date, and
                  (viii) no other compensation, severance or other benefits,
                  except only that this provision shall not limit any benefits
                  otherwise available to Executive under Section 6(c) in the
                  case of a termination following a Change in Control.
                  Notwithstanding the foregoing, however, the Company shall not
                  be required to continue to pay the salary or bonus specified
                  in clauses (i)(iii) or (iv) hereof for any period following
                  the Termination Date if Executive violates the noncompetition
                  agreement set forth in Section 11.

      (b) Section 6(c)(i) of the Employment Agreement is deleted and the
following is substituted therefor:

      (c) Change in Control.

            (i)   Benefits. If during the term of this Agreement a "Change in
                  Control" occurs (as defined below), then Executive shall be
                  entitled to receive the following: (i) salary and vacation
                  accrued through the date of the Change in Control plus an
                  amount equal to three years of Executive's salary as then in
                  effect, payable immediately upon the Change in Control, (ii)
                  an amount equal to three times Executive's target bonus for
                  the fiscal year in which the Change in Control occurs (as well
                  as any unpaid bonus from the prior fiscal year), all payable
                  immediately upon the Change in Control, (iii) acceleration in
                  full of vesting of all outstanding stock options, TARPS and
                  other equity arrangements subject to vesting and held by
                  Executive (and in this regard, all such options and other
                  exercisable rights held by Executive shall remain exercisable
                  one year following the date of the Change in Control, (iv) (A)
                  continuation of group health benefits at the Company's cost
                  pursuant to the Company's standard programs as in effect from
                  time to time (or at the Company's election substantially
                  similar health benefits as in effect at the Termination Date
                  (if applicable), through a third party carrier) for Executive,
                  his spouse and any children, for three years following the
                  date of the Change in Control (even if Executive ceases
                  employment), and (B) thereafter, to the extent COBRA shall be
                  applicable, continuation of health benefits for such persons
                  at Executive's cost, for a period of 18 months or such longer
                  period as may be applicable under the Company's policies then
                  in effect, provided the Executive makes the


                                        2
<PAGE>
                  appropriate election and payments, and (v) no other
                  compensation, severance or other benefits.

      (c) Section 6(d) of the Employment Agreement is deleted in its entirety
and the following is substituted therefore:

            (d)   Termination for Disability. If at any time during the term of
                  this Agreement other than following a Change in Control to
                  which Section 6(c) applies Executive shall become unable to
                  perform his duties as an employee as a result of incapacity,
                  which gives rise to termination of employment for Disability,
                  then in addition to salary and vacation accrued through the
                  Termination Date, Executive shall be entitled to receive the
                  following: (i) continued salary for a period of three years
                  following the Termination Date, payable in accordance with the
                  Company's regular payroll schedule as in effect from time to
                  time, (ii) at the Termination Date, Executive's minimum target
                  bonus for the fiscal year in which the Termination Date occurs
                  (plus any unpaid bonus from the prior fiscal year), (iii)
                  following the end of the fiscal year in which the Termination
                  Date occurs and management bonuses have been determined, any
                  bonus that would have been payable to Executive under the
                  bonus plan in excess of Executive's target bonus, (iv)
                  acceleration in full of vesting of all outstanding stock
                  options held by Executive (and in this regard, all such
                  options and other exercisable rights held by Executive shall
                  remain exercisable one year following the Termination Date (v)
                  (A) for one and one-half years following the Termination Date,
                  continuation of group health benefits at the Company's cost
                  pursuant to the Company's standard programs as in effect from
                  time to time (or at the Company's election substantially
                  similar health benefits as in effect at the Termination Date,
                  through a third party carrier) for Executive, his spouse and
                  any children, and (B) thereafter, to the extent COBRA shall be
                  applicable to the Company, continuation of health benefits for
                  such persons at Executive's cost, for a period of 18 months or
                  such longer period as may be applicable under the Company's
                  policies then in effect, provided the Executive makes the
                  appropriate election and payments, and (vi) no other
                  compensation, severance or other benefits, except only that
                  this provision shall not limit any benefits otherwise
                  available to Executive under Section 6(c) in the case of a
                  termination following a Change in Control. Notwithstanding the
                  foregoing, however, the Company may deduct from the salary
                  specified in clause (i) hereof the amount of any payments then
                  received by Executive under any disability benefit program
                  maintained by the Company.

      2. No Other Modifications. Except as set forth herein, all of the other
provisions of the Employment Agreement shall remain in full force and effect.
Any capitalized terms not defined in this Agreement shall have the definitions
ascribed in the Employment Agreement.

      3. Governing, Law. This Agreement shall be governed by and construed in
accordance with the laws of the state of Connecticut.


                                        3
<PAGE>
The undersigned have executed this Amendment as of February 1, 2001.




GARTNER, INC.



BY /s/  [ILLEGIBLE]                         /s/ ROBERT E. KNAPP
   --------------------------               --------------------------------
   Its                                      ROBERT E. KNAPP



                                       4