Graphical User Interface(GUI) on computers has evolved over 4 decades. The webpage here captures the GUI evolution over a timeline to give the idea how from bit map GUI to touch screen, highly interactive GUI's have evolved over the years.
Root user login in gnome is disabled on Debian system by default for security reasons. If you want to enable it for specific reasons then follow the below steps : 1. Login into Debian system 2. launch the Root Terminal from Applications -> Accessories. 3. Authenticate with password to launch the Root Terminal. 4. Open "/etc/pam.d/gdm3" with vi. #vi /etc/pam.d/gdm3 5. Look for "auth required pam_succeed_if.so user != root quiet_success" , comment it out as below. #auth required pam_succeed_if.so user != root quiet_success 6. Save the modification with Esc+w+q+Enter keystroke. 7. Restart or reboot Debian system. 8. Login with root user in gnome. You should be now able to login successfully.