The GTA franchise has always placed a heavy focus on street and gang culture. Since we're talking about a game satirizing the criminal underworld of urban and suburban USA, this should come as no surprise.
Gangs have always been present in GTA games, however they were just kinda there. Sure, you did missions for them, or against them, and they fought over turf, riches or fame if the plot so desired it. But other than that, they were no more than static set pieces. As we all know, that's not how gangs operate in reality. They are in constant conflict with one another and the authorities. And while massive shootouts are hardly realistic either, they're a heck of a lot more GTA.
The aptly named Gang War Reloaded (War-Combat Mod) is a modded mod (meta-mod?) using the Gang War script made by Kerosini as its basis. Basically, this mod makes the gangs and the police a tad more trigger happy. You can trigger the encounters manually, and all you need do is pick the spawn locations of the two opposing sides, define their faction and let it rip.
Currently, the mod has a whopping five variations on what kinds of conflicts you can set up. First up you have the Gangs vs. Cops mode, where all the criminal affiliated factions (Ballas, Families, Vagos & Lost MC) face off against all law enforcement entities (LSPD, Sheriff, Rangers, the Army, FIB, NOOSE...), with the player fighting for the gangs.
The second version is the same as the previously described one, except here the player backs up the cops. Third, you have a rather peculiar scenario with the Lost MC and the player facing off against the Families with only muskets. Then you've got the Families against the Ballas, with the player strengthening the Families, and finally the same as the Families versus Ballas, except with only knives.