Oh so you were able to control the fan in OSX! Smcfancontrol in unable to control both my GPU fans but they are both PCIe GPUs.
In smcfancontrol do you see a fan specifically for the GPU or because of the iMac design it's just the CPU fan cooling both chips?
There are 3 fan controls, ODD, HDD, CPU. I'm guessing ODD is the GPU. I don't really know, obviously. I just set them all to about midway.
Apparently the ODD fan is the one near the CD drive (Optical Disk Drive). I doubt that the iMac has a fan just for the GPU but you did the best thing by raising all their speed.