You need to have imagemagick and ffmpeg installed.

gifcast.zsh

gifcast () {
  mkdir /tmp/gifcast
  ffmpeg -i "$@" -r 10 /tmp/gifcast/gifcast%05d.png
  convert -layers Optimize /tmp/gifcast/gifcast*.png gifcast.gif
  rm /tmp/gifcast/gifcast*.png
  rmdir /tmp/gifcast
}