InformationGDM is the GNOME Display Manager which handles graphical login for GNOME based systems.
Warning messages inform users who are attempting to login to the system of their legal status regarding the system and must include the name of the organization that owns the system and any monitoring policies that are in place.
SolutionEdit or create the file /etc/gdm3/greeter.dconf-defaults and add the following:
banner-message-text='Authorized uses only. All activity may be monitored and reported.'
Additional options and sections may appear in the /etc/dconf/db/gdm.d/01-banner-message file.
If a different GUI login service is in use, consult your documentation and apply an equivalent banner.