Information
GDM is the GNOME Display Manager which handles graphical login for GNOME based systems.
The disable-user-list option controls is a list of users is displayed on the login screen
Rationale:
Displaying the user list eliminates half of the Userid/Password equation that an unauthorized person would need to log on.
Solution
Edit or create the file /etc/gdm3/greeter.dconf-defaults and edit or add the following:
[org/gnome/login-screen]
banner-message-enable=true
banner-message-text='<banner message>'
disable-user-list=true
Run the following command to re-load GDM on the next login or reboot:
# dpkg-reconfigure gdm3
Default Value:
false
Additional Information:
If a different GUI login service is in use and required on the system, consult your documentation to disable displaying the user list