The most promising ways to decline the gambit are 2...d5 and 2...Bc5, but 2...Qe7 and 2...Qh4+ and 2...Nf6 are all playable.
Usually you can play Bc4, or Nc3, or Nf3, or Be2. d3 is playable but tame. Anyway you should prepare for battle with players stronger than you. You surely can beat players weaker than you without any preparation. Also with white you can play anything you like. With black options are narrower.
Not the Falkbeer countergambit (which is probably not bad as well), but the "Falkbeer equalizer" would me put off instantly.
U can learn it in 10s: 2...d5 and taking on f4
Instead of 4. Nf3, play 4. Qf3. If you want a good book on the KG, I recommend the in-depth, 600-page book, The Fischer King's Gambit, by Timothy Taylor.
I don't believe in it. It needs max. a half page to equalize completely.
Yes, Falkbeer is probably the best way to decline the gambit.
Here is an example
I agree that black can equalize, but in the Berlin defence to the Ruy Lopez 1 e4 e5 2 Nf3 Nc6 3 Bb5 Nf6 or the Marshall gambit black also equalizes. For that reason the top grandmasters now have switched to the Italian Giuoco Pianissimo 1 e4 e5 2 Nf3 Nc6 3 Bc4 Bc5 4 d3, where black equalizes as well. Black also equalizes in the Petrov Russian Defence 1 e4 e5 2 Nf3 Nf6.
On the Bishop's Gambit Accepted 1 e4 e5 2 f4 exf4 3 Bc4 it is interesting to note that Stockfish 8 with White defeated Houdini 5 with it at TCEC super final 16 November 2016, although both engines evaluated a -0.50 disadvantage for White. So there must be some deep compensation, that eluded both programs.
Probably better 6. ... Be6 as given above.
Equality (=) is not the problem. But I don't see potential.
There are balanced positions with dynamic potential. This here is pretty static, no latent dynamism. It's equal like French Exchange.
PS: I generally like the overprotection of f4 by means of ...Bd6 as well which makes it hard to gain an advantage at all.
Yes, here is an example with ...Be6 and ...Bd6
There is no white advantage, but it is more dynamic than the French exchange or the Petrov Russian 1 e4 e5 2 Nf3 Nf6, as the pawn structure is asymmetrical and the f-file is semi-open.
Here is a bit of perspective.
On lichess classical games with players up to rating 1800 41% answers 1...e5 to 1 e4. After 1 e4 e5 2 f4 55% accepts 2...exf4, 13% declines 2...Nc6, 13% 2...d6, 10% 2...d5, 3% 2...Bc5.
So if you play 100 games with white, 41 will be King's gambit, of which 23 will be accepted, and 4 will be Falkbeer.
Back to the original question: if you play the King's Indian Attack, all of your white games will be King's Indian Attack. The King's Indian Attack certainly holds more perspective than some lines of the King's Gambit, the Petrov Russian, or the Berlin Defence to the Spanish Ruy Lopez.
Morphy played Evans Gambit, which is heavily analysed.
Tal played Ruy Lopez.
A bit more perspective
"Which is Better for White to Learn, KIA or King's Gambit?
At say the 1500 level to crush other 1500 players and/or to become a 1700 player"
This original question seems wrong.
The opening is to prepare the middle game, the middle game is to prepare the endgame - Capablanca.
So the logical priority would be
1. Endgames
2. Middle game tactics
3. Opening (last and least)
Within openings the logical priority would be
1. Repertoire for black against 1. e4
2. Repertoire for black against 1. d4
3. Repertoire for white (last and least)
Playing the King's Gambit teaches you how to complicate the game in an objectively worse position. That's certainly an important skill to have, but I'm not so sure it's something you need to train in about every other game. ;)
I'd also recommend against playing opening systems, such as the KIA. Such systems might be good for bullet or fast blitz time controls because they allow you to get a lot of quick moves in. But in the long run, they might actually harm your chess improvement because they teach you to ignore your opponents' moves & plans, rather than adapting to them.
If you're mainly looking to get some quick wins (and some maybe not so quick losses), then just go for the King's Gambit (or some other gambit lines, such as the Scotch Gambit or the Evan's Gambit).
But if you're actually looking to improve, you should probably try to get more different position types in your games. In that case, maybe start playing the Ruy Lopez... or even try 1. d4 from time to time. ;)
