"xclock -display" from guest vserver to host?

My host runs an X server on its display :0
The same host has a vserver running (it is an OS-level virtualization).
Host's name is "gentoo" and the guest vserver is "guest_gen."

Now from the vserver I do:
guest_gen / # xclock -display gentoo:0
Error: Can't open display: gentoo:0

gentoo is reachable because:
guest_gen / # ping gentoo
PING gentoo ( 56(84) bytes of data.
64 bytes from gentoo ( icmp_seq=1 ttl=64 time=0.070 ms

and guest_gen can connect to X server on gentoo because:
localhost ~ # xhost
access control disabled, clients can connect from any host

So what is wrong here?
thank you for any answers