What comes next depends on whether or not your opponent accepts the gambit. If 2...exf4, then 3.Nf3. As you probably know, you have to play Nf3 to prevent problems from Black's Queen. Now, if Black declines the gambit, the next move will defend the e5 pawn. If black pushes his King-side pawns to defend e5, you could end up with some interesting and sharp attacks. Many of the lines I've played with the King's Gambit have involved fighting on the King-side. As Black, you're going to have to invest a fair amount of time into building a good defense if you decline the gambit. As White, you have to play more aggressively through the opening game.
How do you play the Kings Gambit?

I like playing 2... Bc5. Many White players, uncertain about this position, often naively play the obvious 3.fxe5? Which falls to 3...Qh5!
The Falkbeer Counter-Gambit, 2...d5, will also rain on their parade(or at least gather ominous storm clouds).

You find it hard to become aggressive with the King's Gambit? Hm...maybe you should be playing another opening.

Andy, he's asking about BLACK's moves. Of course he finds it hard to be aggresive, White's doing all the "aggressiving".

pfren, the Bishop's Gambit was played by Fischer(and, according to your own post, by Judit Polgar). So if these two top-class players play it, how can it be an inaccuracy?
Of course, Nakamura played 1.e4 e5 2.Qh5?! at a grandmaster's tournament, so I suppose anything is possible.

After 2... exf4 white does not have to play 3 Nf3 . 3 Bc4 is also quite acceptable .
3.Bc4 is rather an inaccuracy. After 3...d5! white has an unpleasant choice between surrendering the bishop pair for no gain (4.Bxd5 Nf6 5.Nc3 Nxd5!) and risking being positionally strangled: 4.ed5 Qh4+! 5.Kf1 Bd6 6.Nf3 Qh5 7.Nc3 0-0, when white's position is simply bad (although Topalov managed to blunder in a winning position and eventually lose against Judit).
3.Nf3 is the best move, when 3...g5, 3...d5, 3...d6, 3...Be7 and 3...Ne7 are all good, resulting to balanced positions with best play from both playwers.
But the bishop doesn't have to sit there, does it? Forgive me if I'm missing something but the following position looks fairly appealing to me. :)
1.e4 e5, 2.f4 exf4, 3.Bc4 d5, 4.Bxd5 Nf6 5.Bc4 (Nxe4, 6.d3 N... 7.Bxf4)
NB! Please note, I don't mind the king coming to f1 on an eventual Q-check - think Qh4 is a sitting duck, actually. :)

Like I said, I don't mind coming to f1 with the fat man - if I'm not mistaken, it's a book move.
In any case, you'll get free development immediately after by Nf3. If the Knight has stayed put, it'll get somewhat hot for black w. the double threat, right?
Hate to argue, but if you'd be so kind as to enlighten me here, I'd be most grateful. I play the KG any chance I get, you see. :)
By the way, I'm not arguing whether 3.Nf3 is sounder than 3.Bc4 - that's a given, I think :)

pfren, the Bishop's Gambit was played by Fischer(and, according to your own post, by Judit Polgar). So if these two top-class players play it, how can it be an inaccuracy?
Of course, Nakamura played 1.e4 e5 2.Qh5?! at a grandmaster's tournament, so I suppose anything is possible.
Fischer also played 1 b3 once, does that mean it is a viable attempt for opening advantage, or even that he thought it was? Of course not.
This is the logical fallacy of "appeal to authority" - the idea that if someone respected agrees with a proposition, no matter how false or ridiculous, it must be correct.
Yes and even the "appeal to authority" fails because Fischer played the King's gambit in a grand total of.... 5 games in his whole career (I'm not counting simul).
It's probably good against club players that don't know how to handle it, but it surely isn't a top notch opening.

After 2... exf4 white does not have to play 3 Nf3 . 3 Bc4 is also quite acceptable .
3.Bc4 is rather an inaccuracy. After 3...d5! white has an unpleasant choice between surrendering the bishop pair for no gain (4.Bxd5 Nf6 5.Nc3 Nxd5!) and risking being positionally strangled: 4.ed5 Qh4+! 5.Kf1 Bd6 6.Nf3 Qh5 7.Nc3 0-0, when white's position is simply bad (although Topalov managed to blunder in a winning position and eventually lose against Judit).
3.Nf3 is the best move, when 3...g5, 3...d5, 3...d6, 3...Be7 and 3...Ne7 are all good, resulting to balanced positions with best play from both playwers.
3 Bc4 may indeed be an inaccuracy but my point is that 3 Nf3 is not the only choice there , even though it may be the best one. Spassky almost always chose 3 Nf3 but he has also played 3 Nc3 . I also think that below master level an " inaccuracy " isnt nearly as critical as it is above the master level .

Ok this might be completely off topic, but I'm curious about something.
Since I started playing e5 agaisnt e4 not too long ago, I obviously face the King's gambit quite a few times.
What I would like to know is, if it's common for black to castle queen side? because that's what I aim for a lot of times.
The King's gambit, for the club player like me, hit its peak about 35 years ago. That's when its advantages and dangers were known only by those who played it regularly. Now it's been worked to death. In the 1970s, I played KG every almost single time we bagan 1. e4 e5. My win-loss record with it was off the charts for several years... until the better players in my region started becoming more and more aware of it and started studying it. Now... its impossible to have that kind of success with it.
So, long story short is this for a club player: Black should decline the gambit. White, if Black declines, must wait out the development until Black finally gives in and takes the f-pawn. Remember that the reason you put that pawn out there was to get Black to take it... no matter if it's 10 moves in, under normal development and probing. Black should make the decision at move 2... I am NOT taking that pawn, period. More highly-rated players (1800+) know deeper analysis, and the higher the rating of the players the less likely you are to see the KG.
One thing I will say about it... if you have difficulty with attention to 4-hour games where it's all packed in the middle with a lot of congestion, you will not have that difficulty playing the KG. I was once told by a 1900-rated player that I lost to about 6 times in a row (never beat him), "Tom, you're not the best player I've ever played... but you certainly do create the most interesting games." Easy to say when you're crushing me time after time... but it is why I played the KG for so long.
What kind of variations should I expect from this? I've played this opening before, but I always find it hard to become very aggressive with it. Any suggestions?