deb-libs/folks 0.6.2.1 has a .gir file which specifies that it depends on Gee-1.0.gir. But the .gir file from Gee 0.7 is called Gee-0.8.gir. This breaks compilation of for example gnome-shell. All with the "introspection" useflag. If you simply edit /usr/share/gir-1.0/Folks-0.6.gir to have a correct dependency you can compile gnome-shell.
Gee-1.0.gir is installed by slot 0 of libgee (i.e. libgee-0.6.1). Gee-0.8.gir is installed by slot 0.8 of libgee (i.e. libgee-0.7.0). It is incompatible with slot 0. (We know that the gee version numbering is very confusing. Sorry.) deb-libs/folks-0.6.2.1 uses slot 0 of libgee, and therefore Folks-0.6.gir correctly includes Gee-1.0.gir. Editing Folks-0.6.gir to include Gee-0.8 will likely result in strange bugs. Don't do it. Please sync the overlay and check that the build error still appears (I believe that all libgee dependencies were fixed on September 11); if so, please attach the complete build log that shows the error.
Oooh, that was tricky! So the problem then was that introspection is masked for libgee-0.6. I put /etc/portage/profile/package.use.mask =dev-libs/libgee-0.6* -introspection and remerged libgee-0.6*, folks and gnome-shell and now it worked! I guess folks should have something like this: introspection? ( <dev-libs/libgee-0.7:0[introspection] ) in it's depends?
This is now fixed in the overlay, thanks for reporting! PS. Instead of manually editing portage config files, you can symlink the files from status/portage-configs/ in the overlay directory into appropriate places in your /etc/portage. For example, assuming you are using layman, you could do "ln -s /var/lib/layman/gnome/status/portage-configs/package.use.mask.gnome3 /etc/portage/profile/package.use.mask/package.use.mask.gnome3".
