When I start my PC with Gentoo everything goes ok until it reaches
INIT: Entering runlevel: 3 ... Starting D-BUS system messagebus
It just stops booting here.
What might be wrong and how to fix it?
That most probably mean dbus did not daemonize. Take a peek at /etc/init.d/dbus and figure out how to force dbus to daemonize.
Should look something like: