Antagonists are the main villians of the Grand Theft Auto Games.
The Traditional antagonist is a powerful Crime Lord in the game and is sometimes even close friends with the protagonist until they betray the player for one reason or another. Typical antagonist timeline consists of the player and antagonist having a business relationship, the antagonist betraying the player for money or another business, the player spending the remainder of the game battling the antagonist's forces while increasing their own gang power, and finally killing the antagonist in the game's climax.
Similar to the Antagonist is the Antagonist Group, which is the gang headed (or at least a powerful member) by the Antagonist. The Player will battle the antagonist group for the game and it will usually be disbanded and make no more appearances. The Antagonist Group will attack the player on sight and, like other gangs, roam the streets of their territory in groups of up to about 6 men.
List of Notable Antagonist
Frank Tenpenny Game: Grand Theft Auto: San Andreas Antagonist Group: Tenpenny is/was a corrupt police officer for the city of Los Santos, San Andreas and therefore is not an offiicial "member" of a certain gang, but the most likely candidate for an antagonist group are the Ballas.
Jerry Martinez Game: Grand Theft Auto: Vice City Stories Antagonist Group: Similar to Tenpenny, Martinez is a corrupt Sergeant for the United States Army and is not in any gang. He does however have control over some gangsters in Vice City that try to kill protagonist Vic Vance, but they are not an official gang. But the Mendez Cartel, headed by the Mendez Brothers, is basically the Antagonist Group.
Dimitri Rascalov Game: Grand Theft Auto IV Antagonist Group: Russian Mafia Sidenote- Assuming the Player exacts revenge on Rascalov before the game's climax and kills him, the Antagonist group will become the Pegorino Family, headed by Jimmy Pegorino.