How to set gnome-fallback as the default session for gnome3 & gdm3.
The easiest way to do it in debian is to use update-alternatives to "install" a new alternative x-session-manager.
update-alternatives --install /usr/bin/x-session-manager x-session-manager \ /usr/bin/gnome-session-fallback 100 --slave /usr/share/man/man1/x-session-manager.1.gz \ x-session-manager.1.gz /usr/share/man/man1/gnome-session.1.gz
This gives gnome-session-fallback a much higher priority than gnome-session so is unlikely to be bumped by any other session managers that might get installed/upgraded