Greetings,
If you don't have a supported CPU with your current BIOS rev, flashing is not possible. Before you proceed, I'd recheck all connections, test with a single DIMM installed, etc to rule out / confirm another issue.
Then, I'd contact the retailer where purchased and ask for their help getting your BIOS updated. Unfortunately, there isn't a way around this. Borrow a supported CPU, etc. Gigabyte support might allow you to RMA for update, but it would probably be quicker if you did it yourself.