I've read about many of the basic variations of the KGD, e.g. these but none of of them mention the following from Black. This suggests to me that this move may be a mistake from black that I could take advantage of, but I see nowhere good for white to go from here.
What should white play next?
[fen "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"] 1. e4 e5 2.f4 d6 3. Nf3 f5