BBB does not detect display

I wanted to connect my display (VGA) to my BBB, and therefore I followed the explanations on the wiki. But when it told me that I have to enter "export XAUTHORITY=ls /var/run/gdm/auth-for-root-*/database", I wanted to see what is in this folder. But this folder does not exist. Is that a bug, or something else? (/var/run exists, but no gdm in this folders…)