There was that Troublemaker bike gang, and they were all armed with pistols. Besides, pistols could be the main wepon for SOME units, but gangs have been known to use assault weapons, SMGs, etc.
I think that if your gonna release a gang addon you shoul equip some of them with pistols (like these guys are only street members, they're carrying guns but it's more of a self-defence thing) and equip some of them with M16s, AKs, UZIs, etc. (like these are the guys that are actually going out to cause trouble)