gdm-3.4.1's pam configs currently do not have a path that invokes pam_systemd.so on login. This causes nm-applet to be unable to communicate with NetworkManager under gnome-shell when USE=systemd is present, causing gnome-shell to panic. /etc/pam.d/gdm needs to have -session optional pam_systemd.so or session include system-login IIRC gdm-3.2.x is affected as well.
After some experimentation, it seems the proper file that needs the change is gdm-password. I assume the same may need to be repeated for gdm-smartcard, gdm-autologin, and gdm-fingerprint.
Should be fixed in 3.4.1-r1, thanks for reporting. >*gdm-3.4.1-r1 (12 Sep 2012) > > 12 Sep 2012; Alexandre Rostovtsev <tetromino@gentoo.org> +files/3.4.1/gdm, > +files/3.4.1/gdm.service, +files/3.4.1/gdm-autologin, > +files/3.4.1/gdm-fingerprint, +files/3.4.1/gdm-password, > +files/3.4.1/gdm-welcome, +files/49-keychain-r1, +files/50-ssh-agent-r1, > +gdm-3.4.1-r1.ebuild, +files/gdm-3.4.1-dconf-0.13.patch, > +files/3.4.1/gdm-smartcard, metadata.xml: > Add support for dconf-0.13.x. Use pam_systemd.so if USE=systemd (bug #434798, > thanks to Yang Zhao). Make libaudit non-automagic (bug #408063, thanks to > Justin Lecher). Use pam_ldap in the greeter if USE=ldap to allow ldap users > to be listed (bug #430740, thanks to Plopmaster). Silence errors in > 49-keychain and 50-ssh-agent (bug #353068, thanks to Ian Abbott).
Thanks for the quick turn-around. Verified.