Adding my experience here. I had a 2011 27 iMac’s GPU go bad with the pink vertical lines. I’m in rural South Korea, so there weren’t a lot of options. I considered baking, but longevity was an issue. This discussion and others like it helped me along. I got this Dell/Alienware GPU from Ebay (below, Apple on the left; new card on the right, note the two silver squares that have “R22” printed on them … those are too high for the heatsink and it’s not a perfect fit) and installed that with iFixit instructions (although there are no instructions specifically for a GPU swap on this model, the dual-drive install instructions got me where I needed to go. The card is the correct size, but two parts on the card were too tall and left a big gap when the heatsink was put back on (below). I salvaged two copper squares from the heatsink of an Apple laptop and used them as spacers with a generous helpings of Arctic Silver between each layer. After that, I screwed the heatsink on tightly. It sits a degree or two...

"Note that the shorter section of contacts should be on the left when installing new modules."

Mine is flipped so the shorter section of the RAM module is on the right.