h6 is the right move. F6 doesn't stop White from sacrificing on f7 it only puts a pawn on f6 which stops Black from playing Nf6 later among other things.
What is the correct way to slaughter the knight in king's gambit?
Here's some analysis of this variation. http://tws27.50webs.com/chess/kings_gambit/allgaier.htm One suggested move is f3 to make the idea of f6 work. It shows f6 failing to the immediage Qxg4.
h6 is the right move. F6 doesn't stop White from sacrificing on f7 it only puts a pawn on f6 which stops Black from playing Nf6 later among other things.
Not that I would like to disagree with tmkroll as he is one of the few subjetive posters to King's Gambit threads but he is not quite right.
The issue with 5...f6 is that it chronically weakings the white sqaures around the Black King. Tmkroll is quite right in the respect that white can still play 6.Nf7 but I think he underestimates the worth of the pawn on f6 which it would seem does, for the time being, bog up the f file seriously denting the worth of white trying try to play 0-0 & Bxf4
I think white has better resource after 5...f6 in just playing 6. Qxg4 immediately.
And that white squared weakness of Black's King side is just too much.
Of course you're right, Hadron, however I posted the link with Qxg4 analysis about 2 minutes after my first post already. I suppose some analysis of White sacking on f7 anyway and the f file being too bogged up might demonstrate what was wrong with my first impulse but we're not disagreeing here.
Ng5 is a dubious gambit. the best move for white is Ne5.
King's gambit is dubious chess in the first place. So yes!
Whether or not you think the entire KG is dubious you have to agree Ng5 is dubious in that position. The main move, Ne5 is much better as Morphysrevenges suggested but seemingly off topic for this thread. (The weebly site I linked to seems to suggest White has practical chances after Ng5 because the opponent might not be booked. That is hope chess and not a good attitude to progress but maybe he's right.)
Here is the position: