I think the King's Gambit is really old-fashioned. But never the less it is a nice opening.
After 1.) e4 e5 2.) f4 black has 3 lines to DECLINE the f-pawn:
Bc5, d6, and f6 (I know what you're thinking about f6...)
Now if you guessed, f6 fails to the following manner:
Now there is two other lines to go with:
Bc5, and d6
These two moves are the good ones. They both have a strong purpose, but their purposes are vastly different.
Bc5 favors a lead in development, and d6 chooses to defend its pawn immediately so it doesn't have to be bothered about later.
(Bc5 diagram)
(d6 diagram)