San Andreas, 1982.
Mike Toreno came back from some deal in Belgium and God knows why FBI deleted him from their membership. However, Mike doesn't want to work there. He goes home and sees... his wife is killed a minute ago and a maniac stands near.
Toreno returns to FBI job to know, who sent that maniac, but soon he opens that 7 gangs are banging the city and forgets about any vengeance, because he has to save not only his life...

Gangs

Girls
Leader: -
Main car: Rally type Truck "Puma"
Carcolors: Pink & White
What Do They Do: on-the-road attacks, killing VIPs, robbery
Ammunition: Machine guns, Shotgun & Silenced Pistol
Cars: outroaders
Places of activity: outskirts and Las-Venturas

Countryside
Leader: Melvin "Digger"
Main car: "Meatmaker"
Carcolors: Grey & Green
What Do They Do: ambushes, riots, large robberies
Ammunition: what they afford to ;)
Cars: again, only cheap ;)
Places of activity: Counties (certainly) and Los-Santos

Genius (Were in GTA2)
Leader: Meteor
Main Car: Meteor
Carcolors: Dark Blue & Yellow
What Do They Do: they wanna make chemical ammunition
to terrorize Liberty-City (looks like Meteor does not like it)
Ammunition: M16, SPAS 12, and all hi tech ammo.
Cars: Supercars and planes
Place of activity: in 90% it's west half of the state

Bikers (Were in GTA3, GTA: Vice City, GTA: Liberty City, GTA: VCS, GTA: LCS)
Leader: "Bull"
Main Car: -
Carcolors: Blue 99%
What Do They Do: To get rich!
Ammunition: DE, AK-47 and Molotovs
Cars: Use your brains ;)
Place of activity: everywhere

Terners' Mafia
Leaders: Nicolas & Julia Terner
Main Car: "Copter"
Carcolors: Black & Red
What Do They Do: killing VIPs, racket
Ammunition: SMG, Stubby Shotgun & smth more...
Cars: Big
Place of Activity: SF and South LS

And the last gang is corruption Police (secret)