Grand Theft Auto 4 (GTA 4)
the game has been out since April 2008, you really could of found these answers pretty easily.
no gang wars it does not fit with the storyline and there were massive complaints about it after san andreas.
And no you can't call in mercenaries or anything like that.
its the guy that lives up in north in those project buildings, if you decide not to kill him.
im not that far right now, but maybe that is what you would like to have. =)