/usr/share/gdm/greeter-config/10_desktop-base 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 | /desktop/gnome/background/picture_filename /usr/share/images/desktop-base/login-background.svg
/desktop/gnome/background/picture_options zoom
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