I am using Adobe After effects on iMac with AMD Radeon R9 M380 graphic card.
The GPU is grey and i can't enable it.
Is there is solution for it please?
If you are referring to the ray-traced renderer, no you can't. That uses CUDA and you can only use CUDA with NVIDIA cards.
That being said, the ray-traced renderer is considered obsolete, so I wouldn't worry about it at all. Use the new Cinema 4D renderer instead; it will be much faster.
The newly GPU-accelerated effects have their settings in a different area (project settings) and THOSE will work on AMD cards (and should work with yours just fine).