Just a small addition: I was unable to install OS X 10.13 on my Gigabyte GA-Z68X-UD3H-B3 until I updated the BIOS to the beta UEFI version. I tried all 3 methods to update the BIOS (I have no Windows installation, so I used a utility to generate a DOS-bootable USB stick):
Q-Flash: gave a checksum error on the BIOS binary.
EFIFLASH: failed to execute.
So if anyone has trouble updating the BIOS, try all possible methods before giving up. I also had to try 3 separate USB drives to be certain it wasn't a media problem or a defective binary.