If you have already filed your return, you'll need to wait for your tax return to either be rejected or accepted, if it is rejected you can add your Form W-2G, correct any other errors on the return and resubmit it.
If it is accepted, you will need to amend your return to add the Form W-2G.
You cannot e-file an amended return. The form is only filed on paper, so it will need
to be printed and filed by mail.
Please see the
following TurboTax FAQ for
instructions on how to amend your tax return in TurboTax.
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