Exhibit 99(c)



                              Gartner Group, Inc.
                                 P.O. Box 10212
                              56 Top Gallant Road
                               Stamford, CT 06904

                                                                July 16, 1999

R.H. Donnelley Corporation
One Manhattanville Road
Purchase, NY 10577

ACNielsen Corporation
177 Broad Street
Stamford, CT 06901

Dear Sirs:

                  Reference is made to (i) the Distribution Agreement (the "1996
Distribution  Agreement"),  dated  as  of  October  28,  1996,  among  Cognizant
Corporation,  which has been renamed Nielsen Media Research,  Inc. ("NMR"),  The
Dun &  Bradstreet  Corporation,  which  has  been  renamed  the  R.H.  Donnelley
Corporation ("RHD") and ACNielsen Corporation  ("ACNielsen") and (ii) the letter
of undertaking dated June 29, 1998 from IMS Health  Incorporated  ("IMS HEALTH")
to RHD and ACNielsen.  In June 1998, NMR distributed to its  stockholders all of
the  outstanding  shares  of  common  stock  of  IMS  HEALTH  (the  "IMS  HEALTH
Distribution").  IMS HEALTH has  announced  its  intention  to  distribute  (the
"Gartner  Distribution")  to its  stockholders  all of the shares of the Class B
Common  Stock of Gartner  Group,  Inc.  ("Gartner")  that IMS  HEALTH  will hold
following the recapitalization of Gartner contemplated by the Agreement and Plan
of Merger  dated June 17,  1999,  among IMS HEALTH,  Gartner  and GRGI,  INC. In
connection  with the IMS HEALTH  Distribution,  IMS HEALTH  undertook  (the "IMS
HEALTH  Undertaking")  to both RHD and  ACNielsen  to be jointly  and  severally
liable  for all  Cognizant  Liabilities  (as  defined  in the 1996  Distribution
Agreement).  Under  Section  8.9(c)  of  the  1996  Distribution  Agreement,  as
applicable to IMS HEALTH pursuant to the IMS HEALTH Undertaking,  IMS HEALTH may
not make a distribution  such as the Gartner  Distribution  unless it causes the
distributed  entity to  undertake  to both RHD and  ACNielsen  to be jointly and
severally  liable  for all  Cognizant  Liabilities  under the 1996  Distribution
Agreement. Therefore, in accordance with Section 8.9(c) of the 1996 Distribution
Agreement and intending to be legally bound hereby, from and after the effective
time  of the  Gartner  Distribution,  Gartner  undertakes  to  each  of RHD  and
ACNielsen to be jointly and severally liable for all Cognizant Liabilities under
the 1996 Distribution Agreement.

         Very truly yours,

         GARTNER GROUP, INC.


     By: /s/ Michael Fleisher
         --------------------
          Name:    Michael Fleisher
          Title:   CFO, Executive Vice President