Apologies if this has been already covered. I’ve been experimenting with gimp-2.99.16 and qt6. Trying to develop filters is way beyond my experience or interest, but I find some of the shipped detail filters (usually at default settings) useful.
So I’ve been building only the gimp plugin, linking that to the static library, and then installing the plugin. For this, invoking qmake seems to do an adequate job - only ‘adequate’ because it isn’t very parallel 
For those who want the command line gmic with gmic-qt and the plugin Arch linux have a (much more complicated) full build using cmake.
Anyway, my purpose in posting this is to say “thanks for porting it to qt6”.