<PAGE>   1

                       SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C.  20549


                                  SCHEDULE 13G


                   UNDER THE SECURITIES EXCHANGE ACT OF 1934
                              (Amendment No. 8)*




                                 GenCorp Inc.
  -----------------------------------------------------------------------------
                                (Name of Issuer)



                         Common Stock, $.10 par value
  -----------------------------------------------------------------------------
                         (Title of Class of Securities)


                                 368682 10 0
  -----------------------------------------------------------------------------
                                 (Cusip Number)




Check the following if a fee is being paid with this statement [  ]. (A fee is
not required only if the filing person: (1) has a previous statement on file
reporting beneficial ownership of more than five percent of the class of
securities described in Item 1; and (2) has filed no amendment subsequent
thereto reporting beneficial ownership of five percent or less of such class.)
(See Rule 13d-7.)


*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 in 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).





                      (Continued on the following page(s))
                               Page 1 of 5 Pages



<PAGE>   2
                                                SCHEDULE 13G

<TABLE>
 CUSIP NO. 368682 10 0                                                                 Page 2 of 5 Pages
<S>     <C>               

- -----------------------------------------------------------------------------------------------------------
|   1 | NAME OF REPORTING PERSON                                                                          |
|     | S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON                                                 |
|     | GenCorp Retirement Savings Plan                                                                   |
|     | (previously GenCorp Savings Plan)                                                                 |
|     | I.R.S. Employer Identification No. 25-6321453                                                     | 
|-----|---------------------------------------------------------------------------------------------------|
|   2 | CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*                                       (a) [ x ] |
|     |                                                                                              ---- |
|     |                                                                                         (b) [   ] |
|     |                                                                                              ---- |
|-----|---------------------------------------------------------------------------------------------------|
|   3 | SEC USE ONLY                                                                                      |
|     |                                                                                                   |
|-----|---------------------------------------------------------------------------------------------------|
|   4 | CITIZENSHIP OR PLACE OF ORGANIZATION                                                              |
|     |                                                                                                   |
|     | Fairlawn, Ohio                                                                                    |
|     |                                                                                                   |
|     |                                                                                                   |
|-----|---------------------------------------------------------------------------------------------------|
|            NUMBER OF          |     5 |  SOLE VOTING POWER                                              |
|                               |       |                                                                 |
|             SHARES            |       |  None                                                           |
|                               |-------|-----------------------------------------------------------------|
|          BENEFICIALLY         |     6 |  SHARED VOTING POWER                                            |
|                               |       |                                                                 |
|            OWNED BY           |       |  5,275,573                                                      |
|                               |-------|-----------------------------------------------------------------|
|              EACH             |     7 |  SOLE DISPOSITIVE POWER                                         |
|                               |       |                                                                 |
|            REPORTING          |       |                                                                 |
|                               |       |  None                                                           |
|             PERSON            |-------|-----------------------------------------------------------------|
|                               |     8 |  SHARED DISPOSITIVE POWER                                       |
|              WITH             |       |                                                                 |
|                               |       |  None                                                           |
|-------------------------------|-------|-----------------------------------------------------------------|
|   9 | AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON                                      |
|     |                                                                                                   |
|     |  5,275,573                                                                                        |
|-----|---------------------------------------------------------------------------------------------------|
|  10 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN                                    |
|     | SHARES*                                                                                           | 
|     |           Not Applicable                                                                          | 
|-----|---------------------------------------------------------------------------------------------------|
|  11 | PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)                                                 |
|     |                                                                                                   |
|     |        15.8%                                                                                      |
|-----|---------------------------------------------------------------------------------------------------|
|  12 | TYPE OF REPORTING PERSON*                                                                         |
|     |                                                                                                   |
|     |        EP                                                                                         |
- -----------------------------------------------------------------------------------------------------------

                                     *SEE INSTRUCTION BEFORE FILLING OUT!                                

</TABLE>



<PAGE>   3
                                                               Page 3 of 5 pages

                                  SCHEDULE 13G
                                  ------------


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

Item 1(b).         Address of Issuer's Principal Executive
- ----------         Offices:
                     175 Ghent Road
                     Fairlawn, Ohio  44333-3300

Item 2(a).         Name of Person Filing:
- ---------           GenCorp Retirement Savings Plan

Item 2(b).         Address of Principal Business Office:
- ---------            175 Ghent Road
                     Fairlawn, Ohio  44333-3300

Item 2(c).         Citizenship:
- ---------           Ohio

Item 2(d).         Title of Class of Securities:
- ---------           Common Stock, $.10 par value

Item 2(e).         CUSIP Number:
- ----------          368682 10 0

Item 3.            If this statement is filed pursuant to Rules 13d-1(b), or
- -------                   13d-2(b), check whether the person filing is a:

                   (a) [ ]  Broker or Dealer registered under Section 15 of the
                            Act

                   (b) [ ]  Bank as defined in Section 3(a)(6) of the Act

                   (c) [ ]  Insurance Company as defined in Section 3(a)(19) of
                            the Act

                   (d) [ ]  Investment Company registered under Section 8 of 
                            the Investment Company Act

                   (e) [ ]  Investment Adviser registered under Section 203 of 
                            the Investment Advisers Act of 1940

                   (f) [X]  Employee Benefit Plan, Pension Fund which is 
                            subject to the provisions of the Employee Retirement
                            Income Security Act of 1974 or Endowment Fund; see
                            240.13d-1(b)(1)(ii)(F)



<PAGE>   4



                                                            Page 4 of 5 pages

                   (g) [ ]  Parent Holding Company, in accordance with 
                            240.13d-1(b)(ii)(G) (Note:  See Item 7)

                   (h) [ ]  Group, in accordance with 240.13d-1(b)(1)(ii)(H)

Item 4.            Ownership:
- -------
                   (a)      Amount Beneficially Owned:
                            5,275,573 shares

                   (b)      Percent of Class:
                            15.8%

                   (c)      Number of shares as to which such person has:
                            (i)     sole power to vote or to direct the vote:
                                    None
                            (ii)    shared power to vote or to direct the vote:
                                    5,275,573
                            (iii)   sole power to dispose or to direct the 
                                    disposition of:  None.  All such power is 
                                    held by Mellon Bank, N.A. (the "Trustee" 
                                    of the Plan).
                            (iv)    shared power to dispose or to direct the 
                                    disposition of:  None.  All such power is 
                                    held by the Trustee of the Plan.

Item 5.            Ownership of Five Percent or Less of a Class:
- ------             Not applicable

Item 6.            Ownership of More Than Five Percent on Behalf of Another 
- -------            Person:
                   The Plan is a voluntary savings plan for eligible employees
                   of GenCorp Inc. and certain of its subsidiaries. Employees
                   who elect to participate in the Plan may select one or more
                   of seven investment options for their contributions, one such
                   option being a fund investing solely in GenCorp shares. All
                   matching company contributions are invested in the GenCorp
                   stock fund as well. Under the terms of the Plan, the Trustee
                   receives dividends on shares held in the fund and is required
                   to invest and reinvest the principal and income of the fund
                   in GenCorp shares. Participating employees ultimately receive
                   such benefits as result from the performance of the fund upon
                   their election to take a distribution of their allocated
                   shares from the fund.

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:
- ------             The Trustee and employees of GenCorp Inc. who participate in
                   the Plan.

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



<PAGE>   5
                                                             Page 5 of 5 pages

Item 10.           Certification:
- -------            By signing below I certify that, to the best of my knowledge
                   and belief, the securities referred to above were acquired in
                   the ordinary course of business and were not acquired for the
                   purpose of and do not have the effect of changing or
                   influencing the control of the issuer of such securities and
                   were not acquired in connection with or as a participant in
                   any transaction having such purposes or effect.

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.

       February 3, 1997
- ------------------------------
          Date

By    /s/ E. R. Dye
- -------------------------------
      E. R. Dye

Secretary of the Administrative
Committee of GenCorp Inc. on behalf
of the GenCorp Retirement Savings Plan
- -----------------------------------------
              Title