With the new version of Gnome 3 on Fedora 21, previous methods of disabling the user list in the gdm login no longer seems to work.
Now you have to edit configuration database with the
gsettings command as the
Type in the following command to set the
disable-user-list option to true in the
gdm user’s copy of the dconf database
sudo -u gdm gsettings set org.gnome.login-screen disable-user-list true
Follow that up with a restart of the gdm service to apply the new setting.
systemctl restart gdm