<PAGE>




                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549


                                  SCHEDULE 13G

                    Under the Securities Exchange Act of 1934

                           (Amendment No. ____2____)*

                              Gilead Sciences Inc.
                              --------------------
                                (Name of Issuer)

                                  Common Stock
                                  ------------
                         (Title of Class of Securities)

                                    37555810
                                    --------
                                 (CUSIP Number)

                                December 31, 1998
                                -----------------
             (Date of Event Which Requires Filing of this Statement)

        Check the appropriate box to designate the rule pursuant to which
                            this Schedule is filed:

                               [X] Rule 13d-1(b)
                               [ ] Rule 13d-1(c)
                               [ ] Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).





                               Page 1 of 4 Pages


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- ---------------------                                           ----------------
CUSIP No.  37555810                     13G                     Page 2 of 4 Page
- ---------------------                                           ----------------

- ---------- ---------------------------------------------------------------------
        1  NAME OF REPORTING PERSON
           S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

           Warburg Pincus Asset Management, Inc.                    13-2673503
- ---------- ---------------------------------------------------------------------
        2  CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)
                                                               (a)  [ ]
                                                               (b)  [ ]
            Not applicable
- ---------- ---------------------------------------------------------------------
        3  SEC USE ONLY

- ---------- ---------------------------------------------------------------------
        4  CITIZENSHIP OR PLACE OF ORGANIZATION

           Delaware
- -------------------- ------ ----------------------------------------------------
                         5  SOLE VOTING POWER

                            Not applicable
                     ------ ----------------------------------------------------
     NUMBER OF           6  SHARED VOTING POWER
      SHARES
   BENEFICIALLY             Not applicable
     OWNED BY        ------ ----------------------------------------------------
       EACH              7  SOLE DISPOSITIVE POWER
     REPORTING
      PERSON                Not Applicable
       WITH
                     ------ ----------------------------------------------------
                         8  SHARED DISPOSITIVE POWER

                            Not applicable
- ---------- ---------------------------------------------------------------------
        9  AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

           Not applicable
- ---------- ---------------------------------------------------------------------
       10  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES
           (SEE INSTRUCTIONS)
           [ ]
           Not Applicable
- ---------- ---------------------------------------------------------------------
       11  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

           Not applicable
- ---------- ---------------------------------------------------------------------
       12  TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)
           IA

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



<PAGE>


Item 1(a):                 Name of Issuer:
- ---------                  ---------------
                           Gilead Sciences Inc.

Item 1(b):                 Address of Issuer's Principal Executive Offices:
- ---------                  -----------------------------------------------
                           535 Lakeside Drive
                           Foster City, CA  94404-1146

Item 2(a)                  Name of Person Filing
- --------                   ---------------------
                           Warburg Pincus Asset Management, Inc.

Item 2(b):                 Address of Principal Business Office:
- ---------                  ------------------------------------
                           466 Lexington Avenue, New York, New York 10017

Item 2(c):                 Citizenship:
- ---------                  -----------
                           Delaware

Item 2(d):                 Title of Class of Securities:
- ---------                  ----------------------------
                           Common Stock

Item 2(e):                 CUSIP Number:
- ---------                  ------------
                           37555810

Item 3:                    If the reporting person is an investment adviser in
- -------                    accordance with Section 240.13d-1(b)(1)(ii)(E),
                           check this box.  [X]

Item 4:                    Ownership:
- ------                     ---------
                           Not applicable

Item 5:                    Ownership of Five Percent or Less of a Class:
- ------                     --------------------------------------------
                           If this statement is being filed to report
                           the fact that as of the date hereof the
                           reporting person has ceased to be the
                           beneficial owner of more than five percent of
                           the class of securities, check the following
                           [X]

Item 6:                    Ownership of More than Five Percent on Behalf of
- ------                     ------------------------------------------------
                           Another Person:
                           --------------
                           Not applicable



                               Page 3 of 4 Pages


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Item 7:                    Identification and Classification of the
- ------                     ----------------------------------------
                           Subsidiary Which Acquired the Security Being
                           --------------------------------------------
                           Reported on By the Parent Holding Company:
                           -----------------------------------------
                           Not Applicable.

Item 8:                    Identification and Classification of
- -------                    ------------------------------------
                           Members of the Group:
                           --------------------
                           Not Applicable.

Item 9:                    Notice of Dissolution of Group:
- -------                    ------------------------------
                           Not Applicable.

Item 10:                   Certification:
- --------                   --------------
                           Inasmuch as the reporting person is no longer
                           the beneficial owner of more than five
                           percent of the number of shares outstanding
                           of the issuer of the securities referenced
                           herein, the reporting person has no further
                           reporting obligation under Section 13(d) of
                           the Act with respect to such issuer.


                                    SIGNATURE
                                    ---------

         After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information set forth in this statement is true, complete and
correct.

Dated:    March 22, 1999



                                           By: /s/ Linda S. Iovan
                                               ----------------------
                                               Name:  Linda S. Iovan
                                               Title: Vice President




                               Page 4 of 4 Pages