I.e. maybe '-auth /var/lib/gdm/:0.Xauth', but I am not sure of the exact location on your distro, use 'ps wwwwwaux | grep auth' to find it.
Also add '-display :0'
So try something like that and see what gdm (or X server) does.
There won't be any. We're looking for messages from gdm why it is exiting/crashing.I don't see "x11vnc" anywhere in the syslog, xorg log, or gdm log.