This is rather an interesting issue. I've had the same problem with both God Of War and Gran Turismo 4. Both are in good state and GoW is still brand-new. Gran Turismo 4 stopped working with time, while GoW did not work at all. I didn't bother with going to exchange it for another, because I thought my PS2 was the problem.
Meanwhile, I've bought several other games, brand-new and used, which work completely fine and raises this issue about GoW. It's rather the PS2 problem or the game disc is defective. And I still need to test my GoW copy in another PS2, tho. :/