An ambitious game to make for a game jam for sure! There is the obvious issue of having people to play a game like this with. Not to mention network issues others have seemed to have. I played a few rounds with the dev as they were kind enough to jump on and let me give it a proper go. I'll leave this point out of the discussion and focus on the game itself.
Firstly, I will say that Outgun Online is a fun little game! A simple but satifying 1v1 2D shooter. The gun selection all feel great to use and the matches flow well!
The game however barely fits the theme, each match having a gimmick that heavily affects things is the premise and it's one that could work rather well, but it didn't feel like anything was different. One change was just low gravity which isn't all that exciting and one was called "big guns" which didn't seem to affect anything, maybe the guns were in fact, bigger, or did more damage, but it wasn't a large enough difference to notice. It just felt like I was playing duck game, but without the ducks.
Despite that, I had a good time with the game and kudos to the dev for making a functioning online game within 48hrs!