What is the best way for black to combat this opening?

EDITED:  2. ...Bc5 (sorry for the typo)

thanks. I've been having trouble with this opening for a while

Korchnoi and Zak demonstrated decades ago that 3...d5 leads to a better endgame for White.  Holding on to the pawn with g5 is good if you are extremely booked up and good on defense.  Declining with ...Bc5 is simple and good.

2...d5 falkbeer counter gambit

You can try the Cunningham: 2. ...exf4 3. Nf3 Be7 (but don't play Bh4 except for rare occasions). But they can always play 3. Bc4 and throw you a curveball.

2...d5 3.exd5 e4! Falkbeer Counter Gambit

I recommend giving up e5 and switching to 1...c5

The "best way" depends a lot on your style and preferences. If you're greedy take the pawn and try hanging on to it at all costs, there are several ways of doing so. If you don't like accepting gambits play...Bc5. If you like counter-gambits play 2...d5 (Falkbeer Counter-Gambit). GM Evans used to say The best way to refute a gambit is to accept it but that may or may not work for you. I rarely accept genuine gambits but I always accept the KG because I've had good luck using Fischer's Defense (3...d6) but again, there are many other defenses available to Black in the KGA.  Whatever you do I suggest you start using the same defense all the time until you have some basis for either abandoning or keeping it in your opening repetoire (ie several results at least). And play through as many GM games as you can using whatever line you decide on playing. You can also look up articles and annotated games with your pet line here and by using a search engine. GOOD LUCK!

In the 1st line mentioned by Melvin Bluestone above instead of 5. Ne5, interesting is Ng5, and if 5...h6 then 6.Nxf7 Kxf7 Qxg4 gives a strong attack for the knight sac, white can continue with moves like Bc4, d4, Bxf4, O-O, Nc3-Nd5 etc (depending on blacks responses) to intensify the pressure.  I have had this a few times in bullet/blitz as white and mashed my opponents easily. 

If 3.Nf3 d5 was a 'practically won game for black' grandmasters would have given up playing it 100 years ago.  Per the Chess.com Game Explorer this move order gives white winning 40.5%, draw 24.8% Black winning 34.7%!

lol...yeah gosh, there have to be some books written on this thing...

I think you ought to have the Kings Gambit Declined in your repetorie.  If you find yourself facing the gambit often, you can also spend the time to learn one of the Kings gambit accepted lines (or a counter gambit), but staying fresh on these lines is hardly worth the effort if you just see it once in a while.

Bc5 would be best

3...d5 is not practically won for black, what kind of nonsense is that.  There is still a whole game to play. 

Oh, that's not complicated at all!!

take the pawn and dont attempt to hold on, instead just develop normaly and focus on forcing whites king to the edge of the board, I have noticed that for some wierd reason castling is frowned upon in the KG unless it is queenside

The main reason you don't see the King's gambit at top levels much, is because after 1.e4 e5 2.f4 exf4 3.Nf3 g5 White has a hell of a time trying to get their pawn back, and often it is Black that is attacking White's king instead with that extra pawn. 3.Bc4 is less common, but probably a better try IMO.

3..Bc5 on the other hand is simpler to learn and probably leads to an equal game.

In either case you'll have to spend a little bit of time learning the correct lines.