In a bug, the maintainer clarified:
so to be clear, gnome-desktop is an old library not used by much anymore. it doesn't have python bindings in the tarball. Those are provided by gnome-python2-desktop.
gnome-desktop3 is actively used (in very limited ways), but it relies on gobject introspection to bind to python.
Note that since gnome-desktop3 uses GObject introspection, it has no own Python code and thus does not show up in PortingDB.