Gun Monkeys is a 2D deathmatch game, though a rather uniquely designed one. Rather than just killing eachother, it's a battle to drop your enemy's energy to 0 while keeping yours above that. In addition to combat, you can collect energy to deposit into your generator and collecting enough will drop your opponent's energy as well. All of this takes place in randomly generated levels which unfortunately aren't always balanced, often only spawning energy on one side of the map or placing one player's generator in a very difficult to use location.
Unfortunately the problems don't end there. For a multiplayer-only game the netcode is terrible with frequent hit detection problems, disappearing energy, and being hit by explosions by entering the area several seconds after it happened. The platforming is well designed, but is often clumsy in practice due to the random generation.
Despite these problems, the game is quite fun. It looks great, has great music, and plenty of player customization to work with. It unfortunately never took off and has very few active players but if you have some friends to play with, it's well worth your time.