There is no real need for GPU, since its not ideal to use GPU for storage. GPU transcoding is better suited for streaming, and it has lower quality than CPU transcoding.
Anyway, just enable it by setting the proper ffmpeg flags as stated in the readme and there you go.
The open source market for smart watches sucks. Gadget bridge doesn't really work for any "new" (i might be wrong here) devices.
Luckly Garmin watches are top notch and the software side its pretty good. You can not install the app and manually download all activities files via USB if you like.
I got put off by how complex that was and that you need to do exactly what you described. So no, I didn't.
I managed telegram tough, and still that was not the most pleasant experience.