The only way to play high games with high response times on an IMac is to get an IMac with high end specs and that'll make it a very expensive gaming machine.
You need to check the sites for the games you play for the nominal (average) hardware specs as well as the higher end hardware specs.
None of the 21 inch screen IMacs will hack it for high end gaming as their GPUs top out at 1 GB VRAM and the i7 CPUs are NOT the fastest available.
The only 5K retina iMac is the 27 inch screen model that starts at $2500 U.S.D. and the 27 inch screen models are the only models to get all of the bells and whistles needed like maybe a top of the line i7 CPU(for games the i7 CPU is really needed over the more common i5 CPUs), top of the line GPU (or see if you can get away with the base (2 GB VRAM GPUs) and faster 7200 RPM hard drives (the 21 inch screen iMacs only come now with laptop standard 5400 RPM hard drives, now) or faster/larger Fusion or SSD/Flash based drives.
You'll be close to the $3000 mark or more if you go with both the fastest CPU (4.0 Ghz) AND the 4 GB VRAM GPU.
If you are trying to compare an iMac gaming machine to a cheap PC for gaming, it sounds as if to me you do not have the budget for a $2500 IMac???
Even the non-retina 27 inch iMacs will set you back, at least $2000 U.S.D. for the higher spec'd. model.
If you are really considering the cheap PC route (which won't be as cheap as you think for a comparable PC with good gaming hardware specs), maybe just stick to your PC for gaming.
It seems to me that the budget is not there or realistic for a IMac with the specs you are going to need.
Good Luck!