When you mess up grub installation somehow, you can restore it using only a Gentoo install cd (seen in http://www.answerbag.com/q_view/142582):

mount /dev/<root partition> /mnt/gentoo
mount -t proc none /mnt/gentoo/proc
mount -o bind /dev /mnt/gentoo/dev
chroot /mnt/gentoo /bin/bash
grep -v rootfs /proc/mounts > /etc/mtab
grub-install /dev/<disc>

Hope it helps