It's come along a fair way - v2.10 has 32-bit per channel improvements amongst other features, though "full" FITS support is still lacking. But, hey, it's free.
https://docs.gimp.org/2.10/en/gimp-i...tory-2-10.html
Apparently, "full" FITS support wouldn't be that hard- NASA already wrote a modern library, it just needs to be added to GIMP's source.