I understand it is a stupid question, because according to Gigabyte's site, the MoBo seems to support every LGA 775 CPU that exists up until today. Although, when I looked at the compatible CPUs section for the specific motherboard, I didn't find the Core 2 Extreme X9100.
Motherboard link:http://www.gigabyte.in/Motherboard/GA-G31M-ES2L-rev-23#ov
CPU list: https://www.gigabyte.com/Ajax/SupportFunction/GetCpuList/?Value=2889&Type=Product
Motherboard link:http://www.gigabyte.in/Motherboard/GA-G31M-ES2L-rev-23#ov
CPU list: https://www.gigabyte.com/Ajax/SupportFunction/GetCpuList/?Value=2889&Type=Product