gdm (at leaast gdm-2.28) needs D-Bus to be started. This should somehow be reflected in xdm init script. (Yes, I know gdm-2.28 is masked :p) Reproducible: Always
Let's cross this bridge when we get to it, i.e. will be fixed when >gdm-2.21 is going to be unmasked.
No problem, just logging it as a known issue.
xorg-server-1.11.2-r2 added "use dbus" to the init script.