What's the damage to the GFID if it's not the contacts?

The GFID's CT primary winding will be rated for a particular max load current, so would likely melt if that were exceeded.

 

The primary "winding" is two brass strips that pass thru the center of the torroid assembly. They were undamaged.The contacts were open and could not be reset. No visible component damage apparent. But given the very compact construction there are really not many components clearly visible.
Given the large number of turns on the secondary, and assuming that the breaker instant trip current is a lot more than it's nominal trip of 15 amps, that secondary pulse was probably over 1000 volts. So that sensor IC, intended to trip when sensing a current differential of about ten milliamps, is probably damaged inside, a bit.