Disable the user list
You can disable the user list shown on the login screen by setting the org.gnome.login-screen.disable-user-list GSettings key.
When the user list is disabled, users need to type their user name and password at the prompt to log in.
Set the org.gnome.login-screen.disable-user-list key
Create the gdm profile which contains the following lines:
user-db:user system-db:gdm file-db:/usr/share/gdm/greeter-dconf-defaults
gdm is the name of a dconf database.
Create a gdm keyfile for machine-wide settings in /etc/dconf/db/gdm.d/00-login-screen:
[org/gnome/login-screen] # Do not show the user list disable-user-list=true
Update the system databases:
# dconf update