Playing GoPro video in Linux
This turns out to be a hassle in some cases using an old LTS Ubuntu. You may find the following command line useful
ffmpeg -i MyVideoIn.mp4 -c:v libx264 -crf 18 -r 25 -s xga -c:a copy OutputVideo.mkv
This will considerably downgrade the video and take some time. It sets the frame rate to 25; 30 might be better but in my case I needed 25. In the end though, the output video works and solves my problem.