gdm3 loads its configuration in its own dconf database. This is separate apparently from the user dconf db that is accessible from the GUI with dconf-editor.
To edit the key value pairs which control the gdm settings and the login screen create a custom file in the /etc/dconf/db/gdm.d and add the custom entries there.
Do the following as the root user:
- touch /etc/dconf/db/gdm.d/01-custom-gdm-settings
Add the following:
Update the dconf database
systemctl restart gdm
More details on this can be found on the gnome sysadmin FAQ.