Both packages share following file: /usr/share/xsessions/gnome.desktop
Gilles committed gdm-2.20.4, are the collisions still there with that release?
(In reply to comment #1) > Gilles committed gdm-2.20.4, are the collisions still there with that release? Yep, it's still there.
ok, gnome-session and the fixed gdm ebuild in tree should fix this for good. Please reopen if it's not the case.