Last move was P to g6 check - either fxg6 or g5-g6. fxg6 could be Q, R, or N, but none of them can retract a move, and black has no other moves to retract [since Pa5 came from c7].
If instead we retract g5-g6+ it is the same story. Black has no move to retract. K from g6 is an impossible double check.
Construction wise, it's better to put bN on g8 instead of a promoted R.
Last move was P to g6 check - either fxg6 or g5-g6. fxg6 could be Q, R, or N, but none of them can retract a move, and black has no other moves to retract [since Pa5 came from c7].
If instead we retract g5-g6+ it is the same story. Black has no move to retract. K from g6 is an impossible double check.
Construction wise, it's better to put bN on g8 instead of a promoted R.
Position is illegal.
Thanks for the tip about g8.