1.8.2 - 1.8.3 - Enable warning banner and disable-user-list

Make sure that /etc/gdm3/greeter.dconf-defaults contains:

[org/gnome/login-screen]
banner-message-enable=true
banner-messsage-text='warning text'
disable-user-list=true

Then reload GDM by running dpkg-reconfigure gdm3.

1.8.4 - Ensure XDCMP is not enabled

Ensure that /etc/gdm3/custom.conf does not contain Enable=true by running cat /etc/gdm3/custom.conf | grep Enable