The top iMac sold today (which happens to still be the 2011 model, if you haven't noticed) outperforms the rMBP in games, particularly if you boost the VRAM to 2GB. The rMBP looks quite pretty, but won't hold up as long as future games are released.
We expect the 2012 iMac, when it's released, to be a beast in games.
Build your own PC for gaming. If you want a Mac, wait for the new iMac. The new GPUs are nearly twice as fast as the current ones. rMBP would probably be the worst choice imaginable. Don't even think of going there.
Maybe the OP just wants a machine that can handle casual PC gaming, in which case, the rMBP or top of the line 2011 iMac, would be fine. I don't get all this talk about "buy a PC" or "build a gaming PC," because quite frankly, PCs are much more trouble than they are worth. Also, the OP can be heavily invested in the Apple ecosystem, which makes the point of buying a PC solely for gaming pointless. Apple machines can easily handle gaming (albeit not at extreme graphical settings even though that is somewhat not the case with Apple's utilization of Nvidia GPUs) and come with a OS that is a joy to use and excel in productivity.