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.
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
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