I have two GPU on my laptop, one is AMD Vega 8 and one is RX 560X.
I run a VOB to MP4 conversion using Windows Powershell with this command:
ffmpeg -i VTS_01_1.VOB -vf yadif -c:v h264_amf -preset slow -crf 19 -c:a aac -b:a 256k output.mp4
Looking at the Task Manager during the conversion what I can see is the Vega 8 GPU is the one being use and not even at max, 10% at most. Not sure if that usage is just merely the operating system using the GPU and not even FFMPEG.