I personally think that there are gangs. They arent really advertised or official, but there are certain parts of the map that are inhabited by certain people. The area with the famous "slingshot swing set" is a great example. there have been times when my car just shot straight forward into a group of guys. They got up and started shooting at me, when i shot back, like 15 other guys started to shoot at me from the street. This can also be said about the mafia guys that drive the tricked out sentinels.
I dont know if this really represents "gangs", but it kinda seems like em to me.