/usr/share/gdm/BuiltInSessions/default.desktop is in gdm3 3.22.3-3+deb9u2.
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 6 | [Desktop Entry]
Name=System X11 Default
Comment=The X11 default session on the system
Exec=default
Icon=
Type=Application
|