Please bear with me Dormilich as I do not run Gentoo. The last time I did, I had help from a friend who is an aficionado of it.
I would recommend you get into your installed system, possibly using the live CD, and get it mounted. Then, see if the instructions on
this page help you with this task.
One bit of credit I must give Gentoo is that there is plenty of documentation for what you need to do, you just need to find it. :) After having it installed at one point, I found it to be quite an amount of work to get it installed, running and configured, yet once it was, it was a SWEET system and ran incredibly well. Once you do get it to where you want it though, you will know a bit of Linux and Administration that you may not have known before. :)
Regards,
Jeff