Great tutorial, I just replaced the GPU of my wife’s late 2009 21.5” iMac with it (found a new one for 30$ on ebay). Some screws and connectors on the logic board have different positions than in this instruction (which is for a 2011 model), but it was still very helpful! Just a hint: I also replaced the PRAM battery (that you see in step 32) while I was at it.

Thanks a lot!