Amazon.com: 2 GB Graphics Video Card GPU Upgrade Replacement for Apple iMac Mid-2011 A1312 27 Inch Core i7 3.4 i5 3.1 Desktop Computer, New AMD Radeon HD 6970M HD6970M 2GB MXM Board
Amazon.com: 2GB Graphics Video Card Replacement for Apple iMac Mid-2011 27 Inch A1312 Core i7 3.4 MD063LL/A MD063 Desktop PC Computer, ATI Radeon HD 6970 GDDR5 2 GB, New GPU Upgrade Repair
Upgrading an iMac GPU for METAL Support! Late 2009 – Mid 2011 Models - YouTube
2011 iMac 27″ GPU failure Fix/Upgrade – JCYCLE
Helping a friend fix up his mid 2011 iMac 27 inch, it seems to have a few problems : r/mac
For iMac 27" A1312 Mid 2009 2010 2011 upgrade NVIDIA K4100M 4GB GPU Video Card | eBay
iMac 2009 to 2011 (A1311/A1312) Upgrade Guide – MacintoshMen