The Gimp allows you to save animations in the GIF format, but I wondered if there were a way to export the animation layers in one PNG, with every layer juxtaposed next to (the right of) the previous one?
Typically you have to find a script to merge all layers into a single one, such as those used to make sprite sheets, for instance this one.
I cannot help with gimp, but the ImageMagick package has lots of command line tools to manipulate images. For example you can split an animated
file.gif into separate
target.*.png files with
convert -coalesce file.gif target.%04d.png
You can then place all these together into a single
montage target.*.png out.png
Example gif (from here), and resulting montage