/usr/share/gdm/greeter-settings/xy_desktop-base.gschema.override is in desktop-base 6.0.7ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [org.gnome.desktop.background]
picture-uri='file:///usr/share/images/desktop-base/login-background.svg'
picture-options='zoom'
|