Hello
I bought my new iMac 27'' 3.2Ghz 8 gb RAM with GT 755m.
I am not a big gamer but I do 3d graphics. In short , no graphic card out there stands a chance however powerful in my hands. I am using the open source 3d app Blender. Blender has a real time render engine that maxes my GPU in a few seconds.
One thing to note here, is that m , that indicated that the version of the GPU is the mobile version, meaning less power consumed (around 50%) but also less performance to the equivalent desktop model (around 2 times).
So if you want to get a computer that will have a very powerful GPU iMacs are definitely not it! A pc will serve you much better. To give you a precise picture , my GPU on this iMAC performs around the same with my pc's at work GTX 650 and that pc is 1 year old.
iMac GPUs are just powerful enough. Another thing to note here is that GPUs has a life of 1 year. 2 -3 years I hear here is very much optimistic even for the most powerful GPUs. But it all depends on the game.
Personally I dont care so much because I know there are people that still play games on Amigas, are those not gamers ? So the whole "Macs are not for games" leaves me rolling my eyes

Sure if we exclude macpro every other mac out there comes with gpus too weak to keep you on the cutting edge of games for more than 1 year. But then there like tons of games out there that are way more forgiving.
I chose iMac because my previous computer was also an iMac and I was very happy with it. The way I see it my new iMac is 8 to 16 times faster than my 7 year old iMac and thats enough for me. I know that if I wanted the best performance at 2000 euros I would have bought a PC. But I love the 27'' screen and I can live with less performance.
An example of comparing the desktop version with the mobile version of 780 can be found here
http://gpuboss.com/gpus/GeForce-GTX-780M-Mac-vs-GeForce-GTX-780