Well, after having done research for the past year or so (I'm ocd a bit in that way), for me the best option is to get a 2012 cMP, which I hope to purchase by the end of the month. As I also do some 3D rendering, I'll be getting a 3.46 GHz 12 core, but most of that won't even be needed for gaming. I plan to also get a GTX 980 Ti (flashed) for the GPU. I'll be dual booting into Windows for gaming. Most posts I've read by people here who have this setup are pretty happy.
For strictly gaming, a 4 or 6 core cMP with a good GPU is all you need.
You don't need a 2012 Mac Pro, for single processor model, 2009 is virtually identical to the 2012 model.
So, get a single processor 2009 Mac Pro (4,1), flash it to 5,1. And then you can upgrade to W3690 / X5690 / X5677.
3x8G RAM should be pretty good for gaming (and working). If you want more, better go for 3x16G rather than 4x8G (the XEON is triple channel, using the 4th slot will degrade the performance a little bit).
For serious gaming, 980Ti is the way to go. However, there is only one reliable source to get the flashed card, and it won't be cheap. Also, without the Pixlas mod, only using internal power to drive the 980Ti is a hit or miss. Both 980 and TitanX run fine with the dual 6pin power source, but 980 seems not always happy with that.
cMP2012 if you want the most expensive and effective route. Otherwise, you are still looking at the high-end iMac.
retina Macbook if you only play solitaire at the professional level.
The cMP route for gaming Mac is not expensive at all. It really depends on what you want.
$400 cMP 4,1 (prefer with the GT120, this is the cheapest option)
$70 3x4G RAM (normal PC RAM is fine for gaming, no need to buy the more expensive server RAM)
$70 X5677 (eBay price, 3.46GHz Quad core, good enough for gaming, no need to go for the expensive X5690)
$250 GTX970 (Just need a good gaming PC card, no need to buy a flashed card. You can simply leave the GT120 in there to provide the boot screen. 970 has very high energy efficiency. If one card is not enough, the cMP has enough power to drive 2x 970 in SLI.)
Overall cost is less than $800, I doubt if you can buy any other good gaming iMac with this price. Besides, the Mac Pro has much better cooling system than other Mac. You can game for 24/7 at very high setting and all the fan still running at around idle.
Even though this price is not including the monitor, however, you can now get a free sync / G sync gaming monitor if you want some good stuff. Or just plug that into your TV for leisure gaming. You have all the choice / upgrade options with the cMP.
Also, this is the only Mac can install up to 6 SATA HDD/SSD internally, plenty of room for storage. Furthermore, AFAIK, the cMP is also the only Mac that can drive the VR at optimum performance without eGPU. All you need is just to buy a good gaming PC card, then it's VR ready.
