I found this guide. Not sure how good it is: https://rog.asus.com/forum/showthread.php?76993-Guide-How-to-install-windows-8-8-1-10-The-proper-way-(UEFI-based-installation)
If you're sure you have GPT, then go ahead and disable CSM in your BIOS. That will make sure you're using UEFI.