It's a good question and I'm not sure there is a very good answer. In your shoes I would either plug a keyboard into the standard keyboard port on the mainboard or get one of those USB to keyboard connector adapters to use with your keyboard while working with the BIOS.
Well i am also using the same keyboard plus the same motherboard
check your chipset drivers
and update your bios to latest version ie 1605
and then have you installed the g15 keyboard software
and if the above doesnt solve the problem this definatly will
borrow a keyboard and then restore defaults settings to bios and then try the g15
The chipset driver or the software will have no effect on it in the bios. To use a USB keyboard in the bios to change settings, you have to enable USB keyboard support with a adaptor or use a PS/2 keyboard. We are not talking about after windows loads.