I have the new 27in iMac with an AMD Radeon R9 M295x. As far as I can tell, I am limited to CPU rendering in After Effects.
I keep reading that GPU is available as long as I don't need CUDA and nVidia for Ray Tracing.
As it is, I'm only seeing CPU available. GPU is grayed out.
Show a screenshot of exactly what you're seeing that has you concerned. From your brief description, it seems that everything is functioning normally.