Brian.... I went and did some digging at the Tire Rack website and checked out what they have for the GTO. When I click on each wheel their web site provides pricing and specs in the left side of the page.
There is some thing called "Bolt Pattern", is that what you are looking for?? It shows 5-120. Click on the prices under the wheels.
From the start of Holden Commodore production in 1978, the standard bolt pattern is five on 120 millimetres. Standard Chevrolet is 5 on 4 3/4" which is 120.65 mm. So, although 2/3 of a millimetre doesn't sound much, a Chev pattern wheel won't fit a Commodore/GTO unless the holes are a sloppy fit, that is. And who likes a sloppy hole??
Guys your best bet is to go to an aftermarket wheel place so you can get a def bolt pattern. This is just me but gettin rims on line is not the best idea. Again just my opinion because the people you order from dont have the car in front of them. Therefore they can mess up and send you the wrong rims. Trust me it has happened to me and it is a Pain in the A$$ to then ship em back etc etc etc. Again do what you like i just feel better about going to a place that can see the car and if they mess up on the rims then its on them to send them back and get the right ones.....Good Luck