I know I’m a bit late in posting this, but better late than never. If anyone has had issues upgrading to GNOME v2.24, and their session manager is acting weird or not acting at all (because it doesn’t work), it’s a good idea to add this mask to your /etc/portage/package.mask file or folder and then re-update your system:
=x11-libs/libxklavier-3.8
The relevant bug report is here.