syslinux

Before anything, you will need to use gdisk to edit a partition and set a legacy MBR bootflag for it. The option is available in the expert list of options in the gdisk menu.

Writing the changes to the MBR will not delete your partitions.

gdisk /dev/sda

Install the new GPT bootloader:

dd bs=440 conv=notrunc count=1 if=/usr/share/syslinux/gptmbr.bin of=/dev/sda

cd /usr/share/syslinux/
cp chain.c32 ldlinux.c32 libcom32.c32 libutil.c32 linux.c32 memdisk menu.c32 vesamenu.c32 /boot/extlinux/

Create a configuration file, and install EXTLINUX.

This file sets the timeout to 60 seconds.

TIMEOUT 600
ONTIMEOUT vmlinuz-3.12.21-gentoo-r1

UI vesamenu.c32
MENU TITLE Boot

LABEL vmlinuz-3.12.21-gentoo-r1
        MENU LABEL Gentoo Linux 3.12.21-gentoo-r1
        LINUX /boot/vmlinuz-3.12.21-gentoo-r1
        APPEND root=/dev/sda1

LABEL vmlinuz-3.10.17-gentoo
        MENU LABEL Gentoo Linux 3.10.17-gentoo
        LINUX /boot/vmlinuz-3.10.17-gentoo
        APPEND root=/dev/sda1

extlinux --install /boot/extlinux/