Help section for GS Gaffer

You are probably missing a codec on your machine to play the selected video.

On a windows machine for example, you may be missing the MPEG2-Video codec to play .mpeg videos.
This would require you to download it and install it, in this case from here:
https://www.microsoft.com/en-us/store/p/mpeg2-video-extension/9n95q1zzpmh4

A google search should reveal what codec you need for a specific format, and let you find a way to download it.