UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the Securities

Exchange Act of 1934

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¨ Preliminary Proxy Statement

¨ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

þ Definitive Additional Materials

¨ Soliciting Material Pursuant to §240.14a-12

GenCorp Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

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*** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Shareholder Meeting to Be Held on March 27, 2013. Meeting Information GENCORP INC. Meeting Type: Annual Meeting For holders as of: January 31, 2013 Date: March 27, 2013 Time: 9:00 AM EDT Location: Omni Berkshire Place 21 East 52nd Street New York, NY 10022 BARCODE GENCORP GENCORP INC. REBECCA A. BAUER P.O. BOX 537012 SACRAMENTO, CA 95853-7012 Investor Address Line 1 Investor Address Line 2 15 12 1 Investor Address Line 3 OF Investor Address Line 4 Investor Address Line 5 2 John Sample 1234 ANYWHERE STREET ANY CITY, ON A1A 1A1 You are receiving this communication because you hold shares in the above named company. This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side). We encourage you to access and review all of the important information contained in the proxy materials before voting. See the reverse side of this notice to obtain proxy materials and voting instructions. Broadridge Internal Use Only Job # Envelope # Sequence # # of # Sequence # 0000156273_1 R1.0.0.51160

—Before You Vote— How to Access the Proxy Materials Proxy Materials Available to VIEW or RECEIVE: 1. Notice & Proxy Statement 2. Annual Report 10-K Wrap How to View Online: Have the information that is printed in the box marked by the arrow g XXXX XXXX XXXX (located on the following page) and visit: www.proxyvote.com. How to Request and Receive a PAPER or E-MAIL Copy: If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request: 1) BY INTERNET: www.proxyvote.com 2) BY TELEPHONE: 1-800-579-1639 3) BY E-MAIL*: sendmaterial@proxyvote.com * If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box marked by the arrow g XXXX XXXX XXXX (located on the following page) in the subject line. Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before March 13, 2013 to facilitate timely delivery. — How To Vote — Please Choose One of the Following Voting Methods Vote In Person: Many shareholder meetings have attendance requirements including, but not limited to, the possession of an attendance ticket issued by the entity holding the meeting. Please check the meeting materials for any special requirements for meeting attendance. At the meeting, you will need to request a ballot to vote these shares. Vote By Internet: To vote now by Internet, go to www.proxyvote.com. Have the information that is printed in the box marked by the arrow g XXXX XXXX XXXX available and follow the instructions. Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card. Internal Use Only 0000156273_2 R1.0.0.51160

Voting items The Board of Directors recommends you vote FOR the following: 1. Election of Directors Nominees 01 Thomas A, Corcoran 02 James R. Henderson 03 Warren G. Lichtenstein 04 David A. Lorber 05 Merrill A. McPeak 06 James H. Perry 07 Scott J. Seymour 08 Martin Turchin The Board of Directors recommends you vote FOR proposals 2, 3 and 4. 2. To consider and approve the Employee Stock Purchase Plan. 3. To consider and approve an advisory resolution regarding the compensation of GenCorp’s Named Executive Officers. 4. To ratify the appointment of PricewaterhouseCoopers LLP, an independent registered public accounting firm, as independent auditors of the Company for the fiscal year ending November 30, 2013. NOTE: To consider and act on such other business as may properly be brought before the meeting or any adjournments or postponements thereof. BARCODE g0000 0000 0000 Broadridge Internal Use Only xxxxxxxxxx xxxxxxxxxx Cusip Job # Envelope # Sequence # # of # Sequence # 0000156273_3 R1.0.0.51160

Reserved for Broadridge Internal Control Information NAME THE COMPANY NAME INC. - COMMON 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS A 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS B 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS C 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS D 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS E 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS F 123,456,789,012.12345 THE COMPANY NAME INC. - 401 K 123,456,789,012.12345 THIS SPACE RESERVED FOR SIGNATURES IF APPLICABLE Broadridge Internal Use Only Job # Envelope # Sequence # # of # Sequence # 0000156273_4 R1.0.0.51160