I don't know OpenSuse but in Debian (since Lenny I think) you need to grab the XAUTHORITY token.
In my current setup I have to lauch x11vnc with gdm's random token.
x11vnc -auth guess
x11vnc -auth /var/run/gdm3/auth-for-Debian-gdm-0Y9dhH/database
Maybe this could help. You have to search for the string XAUTHORITY to find the process allowed to connect to the graphical interface. In otherwords you have to launch x11vnc with gdm's "credentials".
grep --text auth-for /proc/*/environ| cat -v | sed 's/\^@/\n/g' | less