Hello, after noticing that appstream-glib was pulling gtk+3 entirely by itself, I tried to build gimp without (--without-appdata-test) and it compiles and works (a summary "test" was made) as it should. Is this mandatory? If not, please put it behind a USE flag.
It seems to be important for extensions. Could be optional and enabled by default, but upstream doesn't want to make it optional. More info: