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