/usr/share/gnome-session/sessions/gdm.session is in gdm 3.0.4-0ubuntu15.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | [GNOME Session]
Name=Display Manager
RequiredComponents=gdm-simple-greeter;gnome-settings-daemon;polkit-gnome-authentication-agent-1;
RequiredProviders=windowmanager;
DefaultProvider-windowmanager=metacity
|