How to Configure gdm3 to allow root login?

Ubuntu 18.04 comes with GDM (Gnome Display Manager) as a default Display Manager. For Security reason user can’t login with root user.
For development and advanced operation, some user may prefer to login as root.

Follow the steps to enable root login on Ubuntu 18.04 GDM display manager.

Step-1: Modify GDM conf file.

Before doing any changes take a back up:

$ sudo cp /etc/gdm3/custom.conf /etc/gdm3/custom.conf.original

Now open up /etc/gdm3/custom.conf with vi editor as follows:

$ sudo vi /etc/gdm3/custom.conf

Copy and paste the following two lines below [Security] directive.

[Security]

AllowRoot = true

AllowRemoteRoot = true

Save and exit from the editor.

Step-2: Modify PAM conf file.

Now modify PAM (Pluggable Authentication Modules) configuration file.

Before doing any changes take a back up:

$ sudo cp /etc/pam.d/gdm-password /etc/pam.d/gdm-password.original

Now open up /etc/pam.d/gdm-password with vi editor as follows:

$ sudo vi /etc/pam.d/gdm-password

In the file, search for the following line:

auth    required    pam_succeed_if.so user != root quiet_success

To comment the line just add # in front of the line as shown below:

#auth   required    pam_succeed_if.so user != root quiet_success

Save and exit from the editor.

Note: This step was incomplete in the youtube link provided in the post by the Questioner.

Step-3: Reboot

After reboot, you will reach login screen where there will be a display of not listed? option just under username. If you click it, you will get another screen where you can enter root as username and then press next button. Now, the next screen appears which collects password. You enter root password. After successful authentication you will be logged in as root.

Conclusion:

Questioner @Rogério Dec attempts to look for username root from lightdm login screen.

This procedure is for gnome GDM according to the youtube video.

If you have not installed gnome desktop, then install the same using the following command:

$ sudo apt install ubuntu-gnome-desktop

If you have already installed both lightdm as well as gdm3, then you need to switch to gdm through following command:

$ sudo dpkg-reconfigure gdm3

After switched to gdm3, follow step-3 to login as root.