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 reboot.c32 poweroff.c32 rosh.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/
Here's an example of a longer extlinux.conf, with version 6.00 and above:
TIMEOUT 100 ONTIMEOUT gentoo-3.14.24-ck UI menu.c32 MENU TITLE Boot LABEL gentoo-3.14.24-ck MENU LABEL Gentoo Linux 3.14.24-ck LINUX /boot/vmlinuz-3.14.24-ck root=/dev/sda1 quiet LABEL gentoo-ck2 MENU LABEL Gentoo Linux 3.14.4-ck (quiet) LINUX /boot/vmlinuz-3.14.4-ck root=/dev/sda1 quiet LABEL gentoo-ck2-quiet MENU LABEL Gentoo Linux 3.14.4-ck LINUX /boot/vmlinuz-3.14.4-ck root=/dev/sda1 LABEL windows7 MENU LABEL Windows 7 COM32 chain.c32 # APPEND hd1 2 APPEND mbr:1FF400E3-1632-43B4-BDEA-FC2D558E23F0 LABEL reboot MENU LABEL Reboot COM32 reboot.c32 LABEL poweroff MENU LABEL Power off COM32 poweroff.c32 LABEL rosh MENU LABEL Read-only shell COM32 rosh.c32