The current version of After Effects uses the GPU for very little. See this page: GPU (CUDA, OpenGL) features in After Effects
The only really big thing that uses the GPU is the ray-traced renderer. The ray-traced renderer is considered obsolete. I've never used it and I don't know of many professionals who ever have. Most people use the newer, more powerful options that now come with After Effects.
So don't let the consideration of GPU's stop you from purchasing a machine for After Effects.
That being said, these days, I usually recommend iMacs for photographers, but PC's for people working in video. You get a lot more bang for your buck with a PC. And I'm talking actual performance and not just on paper. Windows 10 and Windows 7 both work really well with Premiere Pro and After Effects.
Now, if you are planning to use third-party plugins for After Effects that use the GPU (such as Element 3d), you will want to check their requirements.
Also, Premiere Pro makes extensive use of the GPU too. I'd recommend checking their requirements.
Seriously consider getting a PC. Shop around and look at what you can get for your money.
For AE you want a fast processor clock speed, at least 4 GB of RAM per core of your processor, and an SSD that you can dedicate to AE's cache. If you plan to use Element 3d, get the NVIDIA GTX 970 as your GPU. It's got great performance for the money with Element and it works well with Premiere Pro too.