root user and any GUI

[Jeremy C. Reed]

> Fatal server error:
> Server is already active for display 0
>        If this server is no longer running, remove /tmp/.X0-lock
>        and start again.

So kill that server and kill gdm.

Or run an additional X on a different display (even on same physical 
hardware).

> Xlib: connection to ":0.0" refused by server
> Xlib: Invalid MIT-MAGIC-COOKIE-1 key

This reminds me: also if you don't want to start X as root then instead 
of using xhost to allow root, use xauth and XAUTHORITY.

(As for me, I don't login as root.)