You do not have amend your tax return to file Form 709 - it is filed separately. No attachments or supporting documents are required.
From Instructions for Form 709 United States Gift (and Generation-Skipping Transfer) Tax Return:
File Form 709 at the following address.
Department of the Treasury Internal Revenue Service Center
Kansas City, MO 64999
If using a Private Delivery Service (e.g. Fedex, UPS, etc.), file at this address.
Internal Revenue Service
333 W. Pershing Road Kansas City, MO 64108
Form 709 is an annual return.
Generally, you must file Form 709 no earlier than January 1, but not later than April 15, of the year after the gift was made.
You can extend the due date of filing the return in two ways:
By extending the time to file your income tax return and by filing Form 8892.
