Put the files into C:\Program Files\GIMP 2\share\gimp\2.0\scripts
You'll find them under Filters->Sprite-Sheet
Take any gif and run the "seperateanimation" script.
Every frame of your animation will now contain the entire image.
Then run the "Create from layers.." script and your animation will get converted into a spritesheet.

How to create a gif from a (short) video?
Use ffmpeg with this command:
.\ffmpeg.exe -i .\input.mp4 -filter_complex "[0:v] split [a][b];[a]palettegen [p];[b][p] paletteuse" .\out.gif