Positional assessment at the transition from opening to mid game: White has a ruptured pawn structure: an isolani in the center and awkward pawns on the queen-side; these are potential targets for Black. Black has a good bishop, a solid center, control of the c-file, and no marked weakness for White to target. Black's one backward piece is the knight; Black's next objective will be to get this piece to a better square.
20... Kg8f8 21. g4
I do not know what to make of this move; it seems to be an unnecessary weakening of the king-side pawn structure.
Before the Black knight can be moved, the square e5 must be re-inforced, otherwise a White knight will encroach there.
22. Kg1f1 Nd7b6 23. Kf1e2 Kf8e7 24. Nf3d2
Preventing Black's knight from penetrating the position at c4.
I opted for liquidation at this point, planning to get an active rook to harass the distorted White pawns.
25. Ke2xd2 Nb6c4 26. Ne3xc4 Rc7xc4
White now has problems hanging onto all his pawns.