On Linux systems, AMD GPU support is currently limited to SDR video. No HDR transcoding or tone mapping (PMS 1.32.5 release notes).
There is no official support on Windows. The hooks are in the system, at least for hardware accelerated transcoding. Unsure how well it works.
From Using Hardware Accelerated Streaming:
Note: Our hardware-transcoding system has technical support for many dedicated AMD graphics cards, but we haven’t done official, full testing on those. Support for AMD GPUs is provided “as is” and your mileage may vary. It is recommended that you use Intel Quick Sync Video or a dedicated NVIDIA GPU.
