There are several ways to equalize against the Kings Gambit. However there is no way where black will get a real advantage (with best play from both sides).
So against a player who knows the Kings Gambit as well as you do the positions are equal but tactical and complex. So unless you want to become a top gm the opening is perfectly fine.
Once you start playing the KG often you will notice people will prepare for you and you will get the equality lines.
There are several ways to equalize against the Kings Gambit. However there is no way where black will get a real advantage (with best play from both sides).
So against a player who knows the Kings Gambit as well as you do the positions are equal but tactical and complex. So unless you want to become a top gm the opening is perfectly fine.
Once you start playing the KG often you will notice people will prepare for you and you will get the equality lines.
True. I'm following Simon Williams' Chessbase DVDs on the King's Gambit.
Simon makes great DVD's for your level (<1900ish) he finds a good balance between ideas and moves. Once you get around 1900's you can still use his dvd's but you do have to check his lines because sometimes he is a little to optimistic 
I don't know, it often seems that people who play against the King's gambit are "not quite a little" too optimistic as Black .........
Rybka 3 helped me to discover in 2009 and elaborate 4.Qe2 in the Fischer defense 1.e4 e5 2.f4 ef 3.Nf3 d6 
Black wins a little more often than White in Master games, but the lines of equality are memorized deeply. I taught the K's Gambit's cousin, the out-of-favor-for-60-years Bishop's Opening, to the high-school team I coached in the '90s (before Kasparov played it and it made a small come-back) and they rode it all the way to a County Championship and 3rd in the Scholastic State Tournament.
What's important is that you play and memorize it enough you're ready for anything expected or unexpected - some players won't know what to do and may play anything - even 2...f5 where you better know to play 3 exf5 - not the natural "get the center pawn" 3 fxe5 where, 3 Black moves later, you've lost your h1-Rook!
I play the King's Gambit every game that I get an e5 reply, but I never bother to learn any theory beyond ~3 moves! I have watched one or two of Simon Williams' videos on it though.
Sometimes it leads to into dull lines pretty quickly if they ignore the gambit, but if they accept it can get fiery fast. I've both won and lost many a game in a blazing attack that is all over before move 20 so I always hope people will accept the gambit.
Opponents who don't really know the opening with Black all too often accept the gambit and let you recapture the pawn with no difficulty and get a great centre. Others are rather more stubborn or crafty.
Upon 1.e4 e5, I play 2.f4 in about 8 games of 10, being fond of 1.e4 in general. And upon 1.e4, I play from time to time 1. ... e5 as Black. And every time my opponents play 2.f4, I always accept the gambit 2. ... e5xf4, because that is the only way to try to put problems for White in the King's gambit - that's just my impression after having practiced the King's gambit as White for over 20 years. In case White, after 2. ... ef, within the next 4-5 moves (of course, the mistakes/slips at later stages may lead to disastes, too, but the first moves are of importance - you have to develop your pieces correctly, following some plan - you have to invent the plan whose foundation is build in the opening) makes 1-2 little slips - his position may become hard (if not lost) rather quickly. Much the same concerns Black's play, though.
Here is one of hundreds of examples how not to play the King's gambit as Black: 1.e4 e5 2.f4 ef 3.Nf3 g5 4.h4 g4 5.Ne5 g3? 6.Nxf7. When as White I faced the move 5. ... g3 for the first time (it was a blitz game) - I played 6.Bc4, and even during that blitz game already I had an idea that Nxf7 at once might be interesting. After that game I looked at 6.Nxf7 more closely, and it did prove to be much more interesting than 6.Bc4. It's curious - I faced 5. ... g3 at least 4-5 times even in correspondence games since then - and for that reason OF COURSE then I had desire to explore 6.Nxf7 more deeply, not wasting time to look up that position (after 5. ... g3) on DVD discs or in the reference books or wherever else - even 2800+ chess engine is quite all right for that (being really not sure that I would have found that position somewhere to read about, and really not sure how much time it would have taken me). This is just an example, one of many many more.
It is easy to win if your opponent plays one of the 4 worst moves on the board (Qh4, Qg5 and Ba3 are worst although Ba3 just barely). I also do not understand why any black player in his right mind would play this as it does not develop anything himself but does help white develop.
Decent players will play something like d6 of Nf6 (both leading to equality)
When working on the opening, of course we pay the most attention to opponents' best/good/playable moves (even if their moves are not quite good, but still if I from time to time, or at least once, face such "not good" moves, of course I would like to work at least a bit on those relatively rare moves). I don't know, the exploration of opening variations and elaboration of new opening variations for some reasons is the most interesting sphere in chess for me, of course I do not mean that it is the only interesting sphere for me.
In which position of the King's Gambit is Ba3 even considered as anything beyond just a legal move for Black?!
Maybe it was 1.e4 e5 2.f4 Ba3 meant, or some misspelling by the way, just 2 or three weeks ago, in one of correspondence games, after 1.e4 e5 2.f4, my opponents played 2. ... Bb4).
In the key position after 1.e4 e5 2.f4 ef 3.Nf3 - when speaking about decent playing, Black has about a dozen of moves which lead either to equalty or Black's slight advantage - 3. ... g5, 3. ... d5, 3. ... d6, 3. ... Be7, 3. ... Nf6, 3. ... Nc6, 3. ... h6, 3. ... f5, 3. ... Bb4, and probably several more, like 3. ... g6. In blitz games, as Black, I from time to time play 3.(Nf3) c5!?
I am currently learning the King's Gambit as a regular part of my opening repertoire with 1. e4.
Thoughts and opinions on current state of the opening along with some interesting games and ideas you may have on the opening is always welcome.