mid 2011 21.5" iMac GPU upgrade?

I recently purchased a new computer, but I thought that I might try to turn my old iMac into a gaming computer (mostly out of spite towards people who say that it cannot be done.) Right now my main focus is to start with upgrading the GPU. These are my computers current specs -

Processor 2.8 GHz Intel Core i7

Memory 32 GB 1333 MHz DDR3

Graphics AMD Radeon HD 6770M 512 MB

My question is how would I figure out which GPUs are compatible and not compatible with the computer? Would I need to somehow update my firmware for certain ones? I am not looking to change out the logic board, so the new GPU would have to fit into the socket that is already there. Before anyone tries to spread their ignorance and say that the GPU and CPU aren't replaceable, I'd like to clarify that they are. My iMac is a configure to order model, in which the CPU and GPU are socketed and removable. I greatly appreciate any advice on how to go about selecting a new GPU