You can create a base Gentoo system by using the stage 3 installer. You will also have to mount both dev and proc filesystem for your base system to be usable for chroot. This is the outline of the process.
$ mkdir chroot-gentoo
$ wget ftp://rsync4.jp.gentoo.org/GENTOO/releases/x86/current/stages/stage3-x86-2008.0.tar.bz2
$ tar xf stage3-x86-2008.0.tar.bz2 -C chroot-gentoo
$ sudo mount -o bind /dev chroot-gentoo/dev $ sudo mount -o bind /proc chroot-gentoo/proc
$ sudo chroot chroot-gentoo /bin/bash
$ sudo umount chroot-gentoo/dev chroot-gentoo/proc
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