What are some good defenses against the king's gambit? The king's gambit has a lot of initiative and attack. Is there a good defense to neutralize this initiative?
The best way is Fischer's way. he had a low opinion of it, and so he wrote an entire paper on it's neutralization. Here is the paper and here is his proposal

[Title "King's gambit bust!"]
[StartPly "6"]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

 1. e4 e5
 2. f4 exf4
 3. Nf3 d6


What have we achieved here. 


Our opponent is a pawn down
Our bishop is freed, pointing at the somewhat weakened kingside. With the idea of Bg5 and other threats.
The knight jump on e5 isn't a plan anymore
Lots of ideas for black.