/usr/share/doc/gnome-session: dbus /usr/share/doc/gnome-session-2.24.3: AUTHORS.bz2 ChangeLog.bz2 NEWS.bz2 README.bz2 adding --docdir=/usr/share/doc/${PF} to G2CONF fixed this for me.
gnome-session 2.26 does not suffer this problem and gnome-session 2.24 is bound to stay hard-masked. Closing wontfix.
gnome-session-2.26.1 does for me (USE="doc"): /usr/share/doc/gnome-session-2.26.1/AUTHORS.bz2 /usr/share/doc/gnome-session-2.26.1/ChangeLog.bz2 /usr/share/doc/gnome-session-2.26.1/NEWS.bz2 /usr/share/doc/gnome-session-2.26.1/README.bz2 /usr/share/doc/gnome-session/dbus/gnome-session.html
fixed now. Thanks for reporting.