The "gross annuity amount" is the amount in Box 1. Please see this LINK for more information.
It's odd that there was an amount in Box 2a last year but not this one. Can you try to contact someone at OPM to ask why?
". I was confused when it asked to either select "yes, the taxable annuity amount was used as the taxable amount" or "no, a different amount was taxable." If you answer "yes", it means that you agree that the Box 1 amount is the correct taxable amount. If you answer "no", it is because you believe that the taxable amount is a different amount than box 1.
