Just do a emerge lilo after you followed the installation steps and are going to install grub.
Then create a /etc/lilo.conf en run /sbin/lilo and lilo should be installed. If it doesn't work because you are still in the chroot, then log out of the chroot and copy /mnt/gentoo/sbin/lilo to /sbin/lilo and /mnt/gentoo/etc/lilo.conf to /etc/lilo.conf (I had to do this, because my ataraid device was not in the chroot'ed /dev/ directory).
I still prefer lilo, since grub destroyed my raid twice....
