It is not possible to reverse a Gift Reversal. Your only option would be to re-enter the gift.
You can, however, change the Reversal Date and right click on the gift reversal in the Gift History data grid. This will reveal a number of options. Choose "Change Reversal Date" at the very bottom. Enter the new date and click OK. You might need to process journal entries to move the revenue and assets to a different time period. They do not automatically create for you.