add this line to your /etc/gdm/gdm.conf:
SessionDesktopDir=/etc/dm/Sessions/:/etc/X11/sessions/:/usr/share/xsessions/
Reset your $PREFERRED_WINDOW_MANAGER == gdm and you are good to go.

stop $CURRENT_WINDOW_MANAGER, start gdm

Select GNOME as you session, as usual (you will have to select it or you will get failsafe)

Voila you can make it through the transition period without to much difficulty.