why is 25. ... g6? so bad

 

Black to move

8/1ppr1kp1/2p1p3/p3P2p/P1P2P2/1Pb3P1/4K2P/2B2R2 b - f3 0 25 

The only move here that does not lose for black is 25. ... Kg6.

Why is dat? Black should be able to keep the queenside closed. His focus must be to keep the kingside closed too. To only way to do that is to raise a blockade on the white squares. The only piece that can do that is the black King.

That poses a problem for white. White wants to trade the rooks. He cannot permit black to invade with his rook via the d-file. Hence the rook trade is inevitable.

But once the rooks are traded, which piece is going to drive the black king away from the white squares? The only piece that can chase the black king away from the white squares is the white rook.

So black can set up a blockade on the white squares once the rooks are exchanged, and white cannot make progress anymore.

I'm surprised how clear the position becomes once you have unearthed the right logical narrative!