First, make sure you have the driver 'virtualbox' in your make.conf file for your video card. Then just try to install Xorg. If there are issues, like what I had, take a look at the /etc/portage/package.use file. If it isn't there already you need to add 'x11-base/xorg-server udev' to it. I recommend using the echo command for that, makes things easier and you don't have to use a text editor.
Now, try to install it again. You might need more dependencies or whatever but I didn't luckily. If it installs correctly, try and start it with a 'startx' command. You may or may not get an error saying it cannot find a screen, if so, take a look at the /var/log/Xorg.0.log file(look at this no matter what problem occurs!). Look through it and it will tell you what the problems are. Then just install the things that it is complaining about.
Thats the entire process I took to get Xorg working. Also, when you get it running you will only have like 3 terminals that you can't do anything with, you need to install TWM (and Xclock just for fun!) to actually be able to use the plain Xorg environment.