<DOCUMENT>
<TYPE>424B3
<SEQUENCE>1
<FILENAME>a2086224z424b3.txt
<DESCRIPTION>424B3
<TEXT>
<Page>
                                                FILED PURSUANT TO RULE 424(B)(3)
                                                      REGISTRATION NO. 333-89796

                   5 3/4% CONVERTIBLE SUBORDINATED NOTES DUE 2007

                                  GENCORP INC.

                   PROSPECTUS SUPPLEMENT DATED AUGUST 7, 2002
                       TO PROSPECTUS DATED JULY 15, 2002

    The selling securityholders table on pages 47-49 of the prospectus is hereby
further amended to update the information to include the following entity as a
selling securityholder in the prospectus and to list its total amount of 5 3/4%
Convertible Subordinated Notes due 2007:

<Table>
<Caption>
                                                AGGREGATE
                                             PRINCIPAL AMOUNT                     COMMON         COMMON
                                               OF NOTES AT      PERCENTAGE        STOCK          STOCK
                                              MATURITY THAT      OF NOTES     OWNED PRIOR TO   REGISTERED
NAME                                           MAY BE SOLD      OUTSTANDING     CONVERSION     HEREBY(1)
----                                         ----------------   -----------   --------------   ----------
<S>                                          <C>                <C>           <C>              <C>
Deutsche Bank Securities Inc...............     16,245,000         10.8%            --           881,921
</Table>

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

(1) Assumes conversion of all of the holder's notes at a conversion rate of
    54.2888 shares of common stock per $1,000 principal amount at maturity of
    the notes. This conversion rate will be subject to adjustment as described
    under "Description of the Notes--Conversion Rights." As a result, the amount
    of common stock issuable upon conversion of the notes may increase or
    decrease in the future.

    The preceding table has been prepared based upon information furnished to us
by the selling securityholders named in the table. From time to time, additional
information concerning ownership of the notes and common stock may be known by
certain holders thereof not named in the preceding table, with whom we believe
we have no affiliation. Information about the selling securityholders may change
over time. Any changed information will be set forth in supplements or
amendments to this prospectus.

</TEXT>
</DOCUMENT>