I recently purchased a new i7 iMac with the upgraded graphics card(AMD Radeon R9 M395x).
Now, in after effects preferences > previews section
GPU is not available. I'm shocked after spending the extra $300 for the upgraded graphics card on my new iMac, that GPU isn't compatible with after effects.
Don't be shocked. Your GPU is going to be just fine for most things. It sounds like you are referring to the ray-traced renderer. That feature will never be accelerated by AMD GPUs as it was built on NVIDIA's Optix library. But that's okay because the ray-traced renderer has been considered obsolete by Adobe and hasn't been developed for a few years now. They're leaving it to die as this blog post from 2014 says. And even when it was brand new, I was never that impressed with it. I never used it on a job and I don't know of many who did.
The good news is, the effects that are being GPU-accelerated in the latest versions of AE (Lumetri, Glow, Gaussian Blur, Sharpen, Brightness and Contrast, Find Edges, Hue/Saturation, Mosaic, Tint, and Invert - with more being added in each update) WILL use your GPU. And, most any third-party effects that render on the GPU (Red Giant's Universe and Magic Bullet, Video Copilot's Element 3d, Mettle's ShapeShifter and FreeForm Pro, etc.) will also likely run just fine on it (you'll need to check for specifics, but I don't know why it wouldn't).
So, don't worry! The one, poopy feature won't work, but you have a fine GPU for the actually useful stuff that AE GPU-accelerates.