I haven't tested this mod but it sounds like it might be of some use to you.
You can define factions, the peds that spawn in that faction & choose/restrict the weapons they spawn with in the 'RiotMode_config.txt' file. There is also a Faction Editor tool for it. Should get you some of the way to what you want anyway.
Note: From what I've read it appears the riot peds spawn in from the sky which may be a little immersion breaking depending on what you want.
There is a thread showing it's development here which may contain useful info. That & the mod's comments page are probably worth a read.
Note: It is quite old, last updated May 12, 2015 but there is a good chance it should still work correctly. Test it out & see.
If it's not what you want try searching for 'gang' or 'riot' & other related terms & see if you can find something that allows you to configure factions/peds/weapons etc
There is also this mod although it doesn't have much in the way of options/configuration it does use the ambient peds from the game (no sky spawning immersion break).
Best of luck
I'll look into that thank you, I'm trying to get it to workw ith peds on map editor