> If gnome-session is installed, won't it instead default to GNOME
> instead of the Ubuntu session?
This is a case I tested, and no it won't default to GNOME. Under the covers it will still default to the last enabled session, in that case ubuntu-xorg (because the desktop file is there). The greeter doesn't know of that session though, so it can't tell which session is selected.
> If gnome-session is installed, won't it instead default to GNOME
> instead of the Ubuntu session?
This is a case I tested, and no it won't default to GNOME. Under the covers it will still default to the last enabled session, in that case ubuntu-xorg (because the desktop file is there). The greeter doesn't know of that session though, so it can't tell which session is selected.