My GPU is at 46C while I'm typing this. If the ambient air is 18C, the GPU shouldn't be that hot idle. It's 24C in my room right now. I have my iMac about two feet from the wall.
Fan system sucks air in from the bottom (by the speakers) and puts it out the back at the top. Just make sure everything in the airflow path is clear.
Is the CPU temp also high or is it just your GPU?