The Loonies - The name says it all. Escapees from a funny farm.
Rednecks - Yep, our favorite American people who are the butt of all jokes. Redneck Rampage anyone??
Zaibatzu - Shh, no one is supposed to mention the underground gang called the Zaibatzu. They are in this game most of the time.
Yakuza - What would GTA2 be without different races. This Japanese gang is known for its dealing of illegal drugs.
Hare Krishnas - Remember the orange guys who used to run around chanting in the original? Well they have returned. A religious gang with beliefs slightly different to most mainstream religions. You won't be able to run them all over sadly, because they are now a gang in their own right.
Scientists - As mentioned, GTA2 is set in the future, and science has gone a long way since the 1990's. Cloning is popular, and the Scientists are the results of a mass cloning experiment.
Zloty - The Russian equivalent of the Yakuza gang. They are experienced in the weapons trade.