So here is my proposed "bust" to the King's Bishop's Gambit, with best play by White (main line).
1.e4 e5 2.f4 exf4 3.Bc4 Qh4+ 4.Kf1 Nf6 5.Nf3 Qh6
The "theory" move here is 5...Qh5 instead, presumably to prevent the perceived threat of 6.Ne5. But it turns out to be no threat: 5...Qh6 6.Ne5 d5 7.exd5 Bd6, or 7.Bxd5 Nxd5 8.exd5 Be7, both leave Black a solid pawn up.
The key features of 5...Qh6 are that it 1) protects f4 immediately, leaving Black other, better, options than the weakening g5 in response to a White d3 or d4, and 2) protects e3, at least temporarily, after the Black maneuver N-g4-e3+ Bxe3 fxe3, again giving Black time to gain compensation if White goes after e3.
White's best try here is to accept a slight positional advantage for the gambit pawn:
6.Nc3
Other tries all have good answers by Black: 6.d4 Bb4 (6...Nxd4? 7.Qe2=) 7.Ne5 O-O 8.Nd3 (8.Qf3 Qh4) Nc6. Or 6.Qe2 Be7 7.e5 Ng4. Or 6.Kg1 d6. Or 6.Ne5 covered above. The Ng4 idea comes into play if White plays the weak d3, e.g.: 6.d3 Ng4 7.Nc3 c6 8.Qe1 Bb4 9.Qh4 Qxh4 10.Nxh4 Ne3+ 11.Bxe3 fxe3 12.Re1
6...d6 7.d4 c6 8.Kg1 Nh5 9.Ne1 Nd7 10.Be2 Ndf6
Or 8.Qd2 Nh5 9.Kf2 g5 10.h4 (10.Re1 Bg7;10.Rd1 g4) gxh4 11.Rxh4 Rg8 12.e5 d5 13.Bd3 Be7 14.Rh2 Bg4 15.Ne2 Qg7
