KID against d4 your pick on what to play against e4
Should I learn the King's Indian or Stafford Gambit first?

Both are great, but personally, I prefer the king's Indian, gambits are very likely to be declined on the board or they can have some sort of counter, your opponent usually will give you material back so that you don't get the advantage. I like the king's Indian because it is always a good idea to fianchetto the bishop, it gives a lot of potential to this piece and controls a huge portion of the board, even though it might delay the development.

I link you this video of IM Andras Toth that helped me a lot about these specific topics...
https://www.youtube.com/watch?v=mRgJ_rX9bCI&list=PL4WTyEhy_sF5wQ8o7O2X8WyPdrIIA4asC&index=41

You should learn both at the same time. That is a good way to learn openings. When you play if it was against 1.e4 or 1.d4, analyse your game then look into your openingbook and compare where you messed up. I think this is a good way to do it because you make sure the lines you learn well are lines who actually happen in your games not some line that a GM had in a game 10 years ago
Like Pfren said stafford is not really a good opening but if your goal was to get 1500 online it should work amazing . In a real life chessclub tho they will fall for it once, then they will know the tricks and be very well prepared next time you play them and you will just suffer until you change opening

neither tbh, stafford is garbago and the kings indian defense is only a defense against d4 (you'd have to learn an opening against e5 as well), and its way too complicated for my taste (i dont think it's that good either)

At the 1K level I would learn the Stafford, but only so that you can defend with White against the silly traps.

im probably assuming by "kings indian" you're referring to the Pirc defense, given that you watch Gotham and probably watched his video on the "kings indian system" which is just the pirc defense
theres a lot of difference in this from that
Stafford is bad. It may work in bullet against unprepared opponents. King's Indian Defence is way too complicated for your level. Even Kasparov stopped playing it because it was too much work keeping up with theory. For your level just 1 e4 e5 and 1 d4 d5 are most simple and that is also played up to world championship.
I am watching Eric Rosen and GothamChess atm and wondering whether I should learn the King's Indian or the Stafford Gambit first on my way to 1500.
What do you guys think?