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:

sudo -i

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:

xhost +SI:localuser:gdm

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:

export DISPLAY=:0

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