Hi and welcome to the Gigabyte Forum.
Try using a PS2 keyboard and see if the same problem occurs as this feature wasn't designed for USB keyboards and mice. If you still have the issue we can see if there is some way to fix it.
Can I use a USB to PS2 adapter to accomplish this or should I venture out to find the nearly unfindable PS2 mouse and keyboard to accomplish this. Honestly I think I'd have to order one online as most KB/Mice are PS2 as has been the standard for a long time. This seems really odd that a board of this ilk would not support wake by USB since that has also been a standard for a long time (several MS OS gens). All of the other PCs I have built, owned, bought, or used at work (and believe me they are myriad) have never had this issue. ASUS boards, soyo boards, MSI boards, Dells, HPs, Gateways, etc... none have done this.
I just have a hard time believing this is the final answer on this issue, but I have been wondering if for some strange reason this is the case (that I need to use PS2 peripherals to overcome the problem).