If the rook takes on g4, then 2. fxg4 Rxg4+ 3. Kf3 and the second rook is loose. You either have to play Rg3+, allowing the king to take the pawn and extinguishing your counterplay, or play Qa3+ allowing the capture of the rook. And it really says something about how bad Black's position is in this position that the computer prefers to lose the rook than the pawn.