I came across some GIMP plugins that are for astro processing:
http://www.hennigbuam.de/georg/gimp.html
I have yet to actually use them, despite downloading and installing them some time ago. Seem to remember there being a documentation issue, perhaps I was too lazy to look hard enough at the time.
Pretty sure they're being maintained pretty regularly.
I will get back to it some time soon to learn how to use them.