One Page Rules has a new expansion for Grimdark Future Firefight available to those on their Patreon, but will be letting others get to soon. It's called Gang Wars and as the title suggests, it lets players fight with their gang against their opponent's. Have a look-see.
From the post:
So, what exactly is Gang Wars?
Gang Wars is an expansion for GFF that provides additional rules for violent gang combat in the streets of the dangerous and futuristic Hive City.
The rules are designed to be used both for one-off matches as well as for linked campaigns, and they can be played with any armies, although we recommend using Gangs of Hive City.
This is a standalone expansion, so you just need the free core rules and army lists that are available on the website, however you can also use the extra rules from the Full Rulebook to further expand your games.