for cropping: select a filter "null transform" and click "cropping".
Export as "Animated GIF"
A very good text is: http://www.imagemagick.org/Usage/video/
Basically what you need to do is:
convert source.gif -ordered-dither o8x8,23 -append -format %k info:
and keep making lower/higher the "23" until you find a number just under 256.
convert source.gif -ordered-dither o8x8,23 +map target.gif
And you have a nice full motion movie in GIF in the most optimal compressed format!