As far as GTA 5 mods go, Warfare is pretty straightforward: it turns Los Santos into a militarised urban hellscape, where teams of 20 fight... just because. It does look pretty good though, and once you've installed it there's a fair bit of flexibility involved. You can pit your two teams (yourself and 39 NPCs, in case it isn't clear) against one another in a variety of pre-set Los Santos environments, or else create an arena yourself.
Created by zanzou, the skirmishes involve tanks, choppers and other vehicles, with the arenas ranging from a yacht to downtown Los Santos. There's also a survival mode, which pits you against an infinitely spawning number of baddies. Sounds a bit like life, doesn't it?
You can find instructions on how to install the mod over here.