Download gigabyte windows usb installation tool

http://www.gigabyte.cn/WebPage/-79/usb.html

get Windows 7 SP1 DVD

Windows 7 DVD does not support Skylake

insert Windows 7 SP1 in DVD-ROM

select Windows 7 SP1 DVD and USB flash drive and USB Flash Driver

click start

install Windows 7 SP1 on XHCI

done