Gambits are all about development and attack. Failing to defend against gambits is often about lack of development, and its consequences. Witness the following game.
Wall, Bill - WGMS
1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.Bxf7+
4...Kxf7 5.Nxe5+ Nxe5 6.d4 Bb6
Again, a reasonable line. See "A Reasonable Line".
Not the only line Bill has faced, to wit -
7...Qh4 8.Qf3+ Ke7 (8...Ke8 9.Nc3 Ne7 (9...Bxf2+ 10.Qxf2 Qxf2+ 11.Kxf2 Nh6 12.Nd5 Ng4+ 13.Kg3 Kd8 14.Bg5+ Nf6 15.exf6 h6 16.fxg7+ Ke8 17.gxh8=Q+ Kf7 18.Rhf1+ Kg6 19.Qxh6 checkmate, Wall,B - Shillam, lichess.org, 2016) 10.g3 Qh3 11.Be3 d6 12.Bxb6 cxb6 13.exd6 Ng6 14.Nd5 Qd7 15.Nc7+ Kd8 16.Nxa8 Qxd6 17.Rd1 Ne5 18.Rxd6+ Ke7 19.Qc3 Kxd6 20.Qd4+ Ke6 21.Nc7+ Kf7 22.Qxe5 Rd8 23.Nb5 Bg4 24.Qf4+ Kg8 25.Qxg4 a6 26.Qe6+ Kh8 27.Nd6 Rb8 28.Nf7+ Kg8 29.Nh6+ Kh8 30.Qg8+ Rxg8 31.Nf7 checkmate, Wall,B - Itboss, lichess.org, 2016)
9.Nc3 Nh6 10.Nd5+ Kd8 11.O-O Bd4 12.g3 Qh3 13.Bg5+ Ke8 14.Nxc7 checkmate, Wall,B-Guest6766281, PlayChess.com, 2017;
7...d6 8.O-O Be6 9.Qf3+ Ke8 10.Nc3 a6 11.Be3 dxe5 12.Qh5+ Bf7 13.Qxe5+ Qe7 14.Qxg7 Qf6 15.Bh6 Bd4 16.e5 Qg6 17.Qf8+ Kd7 18.Qxa8 Qxh6 19.Rfd1 c5 20.Qxb7+ Ke8 21.Qb8+ Ke7 22.Ne2 Bh5 23.Nxd4 Bxd1 24.Nf5+ Black resigned, Wall,B - TenAndOnly10, lichess.org, 2016; and
7...Qe7 8.Qf3+ Ke8 9.Nc3 Qxe5 10.O-O Nf6 11.Bf4 Qh5 12.Qg3 d6 13.Qxg7 Rf8 14.Nd5 Qf7 15.Bh6 Nxd5 16.Qxf8+ Qxf8 17.Bxf8 Kxf8 18.exd5 Bf5 19.c3 Re8 20.Rfe1 Be4 21.Rad1 Kf7 22.Kf1 Re5 23.f3 Bg6 24.Rxe5 dxe5 25.Ke2 e4 26.b4 exf3+ 27.Kxf3 Bh5+ 28.g4 Bg6 29.c4 a6 30.d6 cxd6 31.Rxd6 Bb1 32.Rxb6 Bxa2 33.Rxb7+ Ke6 34.Rb6+ Kd7 35.c5 Black resigned, Wall,B - NN, lichess.org, 2016.
8.Qf3+ Ke8 9.O-O Rf8 10.Qh5+ g6 11.Qxh7 g5 12.Qh5+ Rf7 13.Bxg5
Despite Black's dogged Kingside defense, it is likely that White's four pawns are worth more than Black's extra piece.
13...c6 14.Nd2 Qc7
Getting out of the pin on the Knight, but Black needed the freedom that would come with 14...d5.
Targeting the d6 square, which Black overlooks.
15...a5 16.Nd6+ Black resigned
Black can avoid mate only by giving up his Queen.