The python graphics libraries that we use in PyFerret write png files, not gif's. If you do a web search you will find discussions about converting pngs to gifs using ImageMagick. (https://github.com/kohler/
gifsicle/issues/12 ) Is that an option for you? If you or anyone tries it, please let us know how it looks.
convert -delay 100 image???.png animation.gif