iMac 27" Mid 2011 external monitor GPU (ODD) Fan control
55c is not really going to hurt your gpu or your iMac but cooler is always better some people have added a sensor... The other thing is the thermal paste apple used was terrible on all those older iMacs... If you haven't replaced I would definitely do that as well, some of the machines I've played with, the thermal paste was no longer doing anything...It's not really complicated and there are a lot of video guides available if you need them...I would redo cpu and gpu at the same time... That will give you a noticeable reduction in temps...Then when you get your fans set up properly, you should get another drop in temp plus a drop in noise, and probably add a few more years of life to your system. I use Kryonaut thermal grease just a personal preference but there's lot of good stuff out there. the other thing that will help is a good cleaning if it hasn't been opened in a few years... I hope this helps, I really hate typing.