In general, it's just easier to mount an attack against the king with that f pawn out of the way, but if you know what you are doing you can play it with fairly good success. I personally haven't spent much time on the opening, but you might want to look into lines with the Fischer defense (regarding the king's gambit accepted). Hope that helps!
King's Gambit
Hikaru Nakamura plays King's Gambit sometimes. You might want to look at his games. But, you're right. It's not very popular. I'm not sure why, though.
I've experimented with it, but find it to be a very stressful opening. Having that opening Kingside and making sure I can castle (BLACK will often try to prevent it) can be stressful. And if BLACK plays solidly, I'm not sure WHITE has a major advantage oftentimes. Of course, it's always nice when opponents blunder against it. I just prefer something like the Ruy Lopez instead.
Maybe you can post some games of King's Gambit from top players in this thread? That way we can study it more?

I am an avid player of the King's Gambit. I just have some questions, what is wrong with the opening, why isn't it played more, and what are some lines i should be looking to avoid and what not.
LOLZ Oh boy oh boy when the tin foil hat brigade get wind of this they are going to have a right royal field day.
Anywho with your questions I will give it ago
- I am an avid player of the King's Gambit.
Not a question but it does raise one at least. If you are an avid player of the King’s Gambit why are you wanting to ask such questions to begin with?
- What is wrong with the opening?
It depends who you ask. There are few of the aforementioned tin foil hat brigade who frequent this site who believe that, without question, that Fischer’s original fifty plus year old analysis refutes the King’s Gambit out of hand. There are some who cannot move past the apparent fact the King’s Gambit is not played by any titled player of note. Apparently the likes of Nigel Short, Shirov and Nakamura aren’t players of note any more. You even get some who take the time to repeat well-worn dogma from opening manuals on how certain moves (other than Fischer’s 3..d6 that is) can neutralize White’s imitative (of which the ‘Modern’ 3…d5 is the most often quoted). Some are even pragmatic and say “Yes” it is worth ago but you better know what you doing, it does come with some risk. It is with this last group I tend to fall.
- Why isn't it played more?
Played more by who? From a purely statistic point of view a few quick clicks on the on-line chessbase database brought up 56 games played with the King’s Gambit by players 2100 and above. The highest being 2600 something. The results were split roughly 50/50.
From a pragmatic point of view one can only guess that the higher you go the better technique you come up against and also the higher one goes the more you start playing for prize money, norms and to maintain a personal standard. I guess an opening like the King’s Gambit that does entail some risk is not altogether conducive to winning games consistently at higher levels.
- What are some lines i should be looking to avoid and what not.
As White, avoid playing for the Muzio-Polerio Gambit. The Gambit itself is quite playable given certain conditions but Black avoids playing pawn g5 to g4 (to go into the gambit), simply put your boned. Other than that stick to the Kings Knight and King's bishop variations in normal games. Crap like the Breyers, lesser Bishop and Kere's Mason are only fit for fast rapid and blitz.

I am very fond of playing King's Gambit as White. Reading the posts here, I'd add that not only Nakamura plays this opening, I saw also a game where Carlsen played it. And as for Polerio-Muzio, upon 2.f4 ef 3.Nf3 g5 I play 4.h4 only (4. ... g4 5.Ne5).

here's a video of carlsen playing it in a bullet game vs his manager, who is ~2400 Elo
https://www.youtube.com/watch?v=GL-uWmw4YMA
There is nothing "wrong" with it. Just that black as a couple of ways to get a equal position something that is a bit harder in an opening as the Ruy Lopez.
There is nothing wrong with the equal positions from the kingsgambit however. The positions are complex and often the player with the best understanding of the position wins.
That is also the reason it is not often played by GM's. Almost all have a good understanding of any position.

Most of GMs have good understanding not only of KG positions, but positions of other openings, too. If, say, A.Karpov is on the opposite side of your board, it does not matter whether you play King's Gambit or Ruy Lopez
True but that was not my point. My point is that the position is equal but because we are amateurs and do not have any idea what half the positions are about equal does not mattter much in the end result.
For GM's equal means a lot more. They can much better keep the position equal then we can.

Of course ... And as for King's Gambit itself, it seemed to me some 20 years ago it would disappear at the top level, but fortunately it did not

It's really a question - is the top level in chess really measured only by the World Championship level ... ?

I belive black has the most fun after 1. e4 e5 2. f4 d5 3. exd5 exf4. Nothing to back it up, except a personal feeling after a few blitz games.
I will have a guess why it's not played by top players (except a few mad players playing it as a surprise :D), and that is: It gives excellent winning chances for black. A small mistake, and white is toast. In the ruy lopez (for example), if white makes a small mistake, it's equal (maybe even still slightly better for white), and if he makes one more, black gets a small advantage. Not much, where 2 mistakes for white in the KG, and white is simply lost.
Another thing is: black has alot of ways of playing. 2.. d5 with exf4 is good, and white even has to be on his toe to not be worse(!), 2... Bc5 and 2.. exf4 is currently the 3 "accepted" ways for equality. And after exf4, there are several defences for black which gains "mutual chances". Most top players nowadays wants to play for 2 results: Win or draw. It's not worth losing.

I belive black has the most fun after 1. e4 e5 2. f4 d5 3. exd5 exf4. Nothing to back it up, except a personal feeling after a few blitz games.
I will have a guess why it's not played by top players (except a few mad players playing it as a surprise :D), and that is: It gives excellent winning chances for black. A small mistake, and white is toast. In the ruy lopez (for example), if white makes a small mistake, it's equal (maybe even still slightly better for white), and if he makes one more, black gets a small advantage. Not much, where 2 mistakes for white in the KG, and white is simply lost.
Another thing is: black has alot of ways of playing. 2.. d5 with exf4 is good, and white even has to be on his toe to not be worse(!), 2... Bc5 and 2.. exf4 is currently the 3 "accepted" ways for equality. And after exf4, there are several defences for black which gains "mutual chances". Most top players nowadays wants to play for 2 results: Win or draw. It's not worth losing.
That is an interesting reasoning. In King's Gambit, both for White and for Black it may be enough one mistake to get a bad or even a lost position.
Even at the top level, if someone plays several hunderd of Ruy Lopez games, in order not to be fed up with it, it's possible that they from time to time will think of playing something for a change, in that case (1.e4 e5) 2.f4 might be as practicable for that purpose as, say, (1.e4 e5) 2.Nc3 or 2.Bc4 or (1.e4 e5 2.Nf3 Nc6) 3.Bc4, and not only in blitz or training games. Spassky was fond of King's Gambit, and although he liked Ruy Lopez, he played relatively often (for a chess player of that level) the King's Gambit, and even when he got a worse position against Bobby Fischer (Mar del Plata, I don't remember the year of the game now), even at that level (soon the same opponents played the WC match in 1972, as we know) Black should not think that he gets a better or a won position automatically, and it's really quite possible for Black to lose in the King's Gambit, even if a player of Fischer's level plays as Black ...

Quite a few examples in "My System" use the King's Gambit also, so I think it has its merits for those who have studied deep enough into the opening lines to not make any mistakes. The Fried Liver attack was used by Shirov in the Olympiad to beat a 2400 player, and Magnus Carlsen himself used the Scandinavian Defense against Caruana for a win, so I think even "refuted" systems can be used with great success if a player is willing to study the theory in great detail.
Personally, the only time I play f4 in the opening is against the Sicilian, for the Grand Prix attack e4 Nc3 f4.

I love playing against the King's Gambit, the fun line being:
CM JamesColeman destroyed me with that line. A good strategy, and one I missed and so wished I'd seen, is this line for White:

I play this line against King's Gambit, this variant will remove the poison.
Hi! I love the King's gambit.
I think the falkbeer counter-gambit for the KGD, the Cunningham, Bonch-Omolovsky, Kieseritsky, Becker, Schallop, Fischer, and Abbazia for the KGA all have amazing possibilities for both sides and can lead to exciting positions.
So, in the falkbeer line you showed, I believe White playing Bc4 is an innacuracy, and Bb5+! would be a much better line to play as White, as demonstrated above.
Hope you enjoy it :)
I am an avid player of the King's Gambit. I just have some questions, what is wrong with the opening, why isn't it played more, and what are some lines i should be looking to avoid and what not.