Here's Black's first move, Bxg2+:
White has one legal move, Kg1. Black then mates with Gh3:
Here's Black's first move, Nxf3+:
White has two legal moves, Kh1 and Kf1. In either case, Black mates with Gh2. Here's the final position when White plays Kf1:
Here's Black's first move, Ge3+:
White has two legal moves, Kh1 and Kh2. In either case, Black mates with Gf1. Here's the final position when White plays Kh2:
Here's Black's first move, Bxg2+: White has one legal move, Kg1. Black then mates with Gh3: