Alright, if you're absolutely positive your GTA: SA DVD is in perfect condition then the chances are it's a problem with your PS2. I know that's kind of hard to believe considering your other games work fine, but it can't hurt to try it on another person's PS2 if you can. If it still doesn't work, you know it's the game, otherwise you need your PS2 looked at. Chances are it's the laser that needs cleaning, probably nothing serious. If it is the laser you can get that cleaned yourself, there are a lot of sites online that show you how.
Keep us informed of what happens.