Unfortunately, X.org does not work out of the box when installing it inside VirtualBox on a Gentoo guest machine.

The VESA driver didn't really work when starting X, it spitted out the following line:

Well, maybe the VirtualBox Guest Additions will help out. They installed without a glitch on the Gentoo guest, but when starting X I got a similar error as above, only VESA replaced with some VirtualBox stuff.