In the default graphical environment, configuring the login warning banner text
in the GNOME Display Manager's login screen can be configured on the login
screen by setting banner-message-text to 'APPROVED_BANNER'
where APPROVED_BANNER is the approved banner for your environment.
To enable, add or edit banner-message-text to
/etc/gdm3/greeter.dconf-defaults. For example:
[org/gnome/login-screen]
banner-message-text='APPROVED_BANNER'
After the settings have been set, run dconf update.
When entering a warning banner that spans several lines, remember
to begin and end the string with ' and use \n for new lines.