A year is usually how long a game stays on Gamepass. When it comes to rockstar titles, that is not the case. It is usually 3 or 4 months.
GTA3 is unusually short in comparison. It not even going to stay a whole two months on the service since it being added on the 7th.
Red dead redemption 2 stayed on game pass for 4 months
Red dead online was 3 months
GTA5 was 4 months
San Andreas is probably going to be 3 or 4 months if we going by the previous titles.