2011 iMac 21.5” GPU upgrade

With all due respect, the best GPU upgrade for a 2011 iMac is to replace it with a current model iMac. It simply make no sense to spend money on a 14 year old computer assuming you can find the part. Current machines GPUs far out-perform 14 year old technology and can run current versions of Mac OS.

The iMac's CPU's and GPU's are not upgradable to anything other than a few of the OEM chips.

There are two replies to your first inquiry, at > CPU upgrade in 2011 iMac - Apple Community

If you look there, padams35 has been kind enough to share a list of options.