Try using ffmpeg to generate the GIF as well. It's a two step process.
Step 1 Generate a palette
ffmpeg -i ppt.mp4 -vf palettegen palette.png
Step 2 Generate the GIF
ffmpeg -i ppt.mp4 -i palette.png -filter_complex paletteuse out.gif
If the above doesn't produce an acceptable result, we can try using ffmpeg to screengrab the PPT animation and use that as the source.