root login from GUI

Logging in through a graphical user interface as root is considered a security risk, so most display managers aren't going to cater to this. You should almost always use sudo to execute commands as root. If you would like a root shell, then on Debian-based systems such as elementaryOS, log in as your normal user and run sudo su from the terminal. If you need to avoid logging in as a normal user, either boot in to emergency / single-user mode by selecting the option from the Grub menu or switch to a different, non-graphical TTY with Ctrl-Alt-F2, for example, and login that way as root. You can switch back to the display manager with Ctrl-Alt-F7.

When you say "root" do you mean a user named "root" (the default sudo username) or do you just mean logging in with root privileges?

What do you need to do with a root login?

Thanks, Col