Hmm i would say it's a problem with your laser but if your PS2 plays ALL of your other games then that may not be the problem...?
I would say first try giving your disk a clean, it may be that the laser is having trouble reading the disk because it's a little dusty. If it still doesn't work after that, then i would exchange your GOW2 for a new copy. The new copy should definitely work fine if nothing prevails before that.