Radeon Pro 580 iMac Mojave GPU not supported
After Effects CC version 16.0.1 (OS X)
2017 iMac AMD Radeon Pro 580 GPU does not appear to be supported anymore. Effects that require GPU are no longer working.
Thank you for filing this request.
In the screenshot you provided of the GPU Information dialog, the menu that displays only CPU is for the Ray-traced 3D renderer. This renderer is only GPU accelerated for Nvidia GPU’s. Also, as of After Effects 16.0 this renderer has been deprecated and will be removed in a future release; you should use the CINEMA 4D renderer instead.
GPU acceleration for the Ray-traced 3D renderer is separate from GPU acceleration for effects. To enable GPU acceleration for effects, choose File > Project Settings, then click on the Effects and Rendering tab, and set Use to Mercury GPU Acceleration.
GPU acceleration for effects is available on macOS using the Metal or OpenCL GPU technologies. On Windows, supported GPU technologies are OpenCL and CUDA. (CUDA is available with Nvidia cards only.)
If you need additional assistance with this problem, it could be solved more easily by our support team. You can open a case with them via call or chat at this link: https://helpx.adobe.com/contact.html