I had the same thing happen a few days ago using the Gentoo GUI installer. Did you use one of the installers? I think that they still don't work. There is a step by step instruction for doing a stage 3 installation by hand at the Gentoo web site. I believe that it is the only reliable way to install Gentoo. You still need to boot a live CD so you can use the Gentoo live cd. Then you have to open a terminal window, enter sudo su -, and then follow the instructions for a manual stage 3 installation.
If you are using a x86 computer then this is the place to start.
If you have a different architecture then start at the previous page.