The best tables I've played on were GC I and IIs, Diamond Pros, and a Gandy. To my knowledge I've never played on a GC III, IV, or V. I've also never played on a Schmidt. My GC I plays just like the GC IIs at the pool hall. Now that I have one I know better how to look for potential problems. The slate and rails are still in good condition on my GC I so with a little work it still plays great. When you look underneath it does show it's age though. Lots of different screw holes in the wood and a ball box that doesn't exactly fit the table. I'm told the ball box came from a GC II. GC Is were built between 1963 and 1969 so most of them have a lot of history.
First see how it plays, then look underneath. I wouldn't buy one that wasn't put together so I could try it out.