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.

So that didn't work out. My reflex was to Google it. And it pays to look further than the first page of hits, because I hit this page. It talks about a post talking about enabling the CONFIG_VM86 option in the kernel. It seems to be a deprecated option and I don't really have a clue what it does, but make sure the following line appears in the Linux kernel configuration /usr/src/linux/.config:

and recompile the kernel. After the reboot I suddenly saw a big X window in front of me. It worked!