Hi there,
for some reason, still as yet unknown, Gigabyte removed all of the AMI BIOSes for the 6 series boards. So the best advice I can offer is to stick with the award BIOS for now and wait until Gigabyte re-releases them.
I did update to the U1a AMI BIOS for my own motherboard (GA-Z68X-UD3P-B3) and have had lots of reboot, shutdown and other issues with it. So far, I have been able to struggle through and I am quite prepared to beta test but, with hindsight, I wish I hadn't switched to the AMI BIOS so quickly.