GTA1 and GTA2 are free to download under Rockstar Classics http://www.rockstargames.com/classics/, you could try installing them using Dosbox http://www.dosbox.com/ but I'm not sure what the performance will be like. I've done some more investigation and it seems the GTA3 series is indeed Intel Mac only.
True Crime: Streets Of LA is the closest thing to a 3D GTA game out there on the PowerPC platform. And it's trash-cheap on Amazon at the moment. Well worth the pickup.