According to https://bugzilla.gnome.org/show_bug.cgi?id=679438 gobject-introspection should now be python3-compatible. Looking at the git log, it seems most fixes were around 1.47, which means that hopefully 1.48, which is in the portage tree but not yet stable, should work with python3 as well
Just noticed that 1.48.0 was marked stable but still only has use flags for python2.7
I had to fix packagekit-base but this is not in for Gnome 3.22.
Gnome 3.22 is now unmasked. Thanks for reporting.