As the achievement description says: kill 1000 gang members. The three types of gang members are:
Morningstar: Gang members dressed in red and black. They drive around in red and black vehicles with a star labeled somewhere on the hood.
Deckers: Gang members dressed in blue mechanized clothing with katanas strapped to their backs. They drive around in blue and white vehicles labeled with a type of skull graffiti.
Luchadores: Gang members dressed in bright green clothing and resemble Lucha Libre wrestlers. They drive around in bright green vehicles.
Each gang member will appear as a red dot on your mini-map. Just don’t confuse them with police officers. This achievement will come naturally playing through the game.