/usr/share/gnome-session/sessions/gdm-shell.session is in gdm3 3.14.1-7.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [GNOME Session]
Name=Display Manager
RequiredComponents=gnome-shell;gnome-settings-daemon;
|