The Gnome login screen normally shows a list of available users to log in as. For those who want to disable showing the user list, and manually type a username to login with, below I will show you how.
Open terminal by pressing Ctrl+Alt+T on keyboard, when it opens, run following commands one by one:
1. Run command to get access to root:
Type in your password (no visual feedback while typing) when it prompts and hit Enter.
2. In the terminal, run command to allow gdm to make connections to the X server:
3. Then switch to user
gdm, which is required to run gsettings to configure gdm settings.
su gdm -l -s /bin/bash
Update May 2020: For Ubuntu 20.04, you may need to run one more command:
4. Finally hide user list from login screen using Gsettings:
gsettings set org.gnome.login-screen disable-user-list true
After that, restart your computer and enjoy!
How to Restore
To restore the change, open terminal and re-do previous steps, except running the last command with:
gsettings set org.gnome.login-screen disable-user-list false