Windows 7 is not a supported operating system with 7th Gen Kaby Lake CPU. A 6th Gen Skylake CPU is the required CPU for Windows 7 or 8 on the GA-B250M-DS3H.
1.Windows 10 64-bit (for 7th Generation Intel Processors)
2.Windows 10 64-bit / Windows 8.1 64-bit / Windows 7 32-bit / 64-bits (for 6th Generation Intel Processors)
* Please download the "Windows USB Installation Tool" from GIGABYTE's website and install it before installing Windows 7.