My go to commands for processing GoPro footage (or really any video) with ffmpeg:

1. Merge all mp4 files in to a single file without reencoding:

   This is very useful on computers without powerful GPUs (older computers or in my case my Synology NAS)!

ffmpeg -f concat -safe 0 -i <(for f in GX*.MP4; do echo "file '$PWD/$f'"; done) -c copy output.mp4

2. Compress:

for i in GX*.MP4; do ffmpeg -i "$i" -c:a copy -c:v h264 -crf 22 "${i%.}_lowres.mp4"; done

3. Extract audio from a video file:

ffmpeg -i $1 -vn -acodec copy "${1%.*}.aac"