Posted here before for 2.5 on the same issue, just wondering if the workaround with GDM2.20 will work? the link to the previous post is: viewtopic.php?f=66&t=4377
edit, just tried with this result:
munch@munch-laptop:~$ sudo apt-get install gdm-2.20
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package gdm-2.20
munch@munch-laptop:~$ cd /etc/gdm
munch@munch-laptop:/etc/gdm$ sudo sed 's|X11R6/||' gdm.conf >/tmp/gdm.conf
sed: can't read gdm.conf: No such file or directory
munch@munch-laptop:/etc/gdm$ sudo mv /tmp/gdm.conf .