… it started!
The problem was wrong permissions on /var/tmp, so I prepared a new freebsd-baselayout that takes care of fixing /tmp and /var/tmp permissions to be safe.
Note: you cannot just emerge gnome right now, until bug #103124 is closed or I’ll get around adding all the ebuilds to the overlay… that would require a bit of time.
Tomorrow is time of Gnome 2.11.. I hope it will work!