Another question recommended that I use extlinux. It displays the rather unhelpful message Boot error.
Why wouldn't it work? How can I debug the problem?
Disk layout: on /dev/sda rEFIt is installed. /dev/sda4 is / and there is no separate /boot partition.
Method of installation:
extlinux /boot
extlinux.cfg
DEFAULT Gentoo
LABEL Gentoo
KERNEL /boot/kernel
APPEND -