Yes you can install any game on the iMac 5K under Bootcamp. But I think the thing you're after is how well it runs certain games, right? It's best to check a few threads in this forum on this subject and Google/Youtube on 'Gaming performance iMac 5K'.
In general the iMac 5K is very capable of gaming once you BTO it to AMD Radeon R9 M395X. Just don't expect to fully max out games with the native resolution. Many people scale it down to 2560x1440 when gaming.
Categorically not a good PC for gaming. The GPU isn't even as strong as a 7950 and that came out over 3.5 years ago and can currently be bought for about £80