Which Gigabyte board do you have? I have a GA-X58A-UD5 and there are instructions on page 128 on how to configure the Mic.
If you don't have a manual, here's the link for my mine - check out page 128 - GA-X58A-UD5 (rev. 1.0) - GIGABYTE
If you don't have a UD5, look for your manual.
Sorry I don't use my onboard soundcard so I can't quickly say what needs to be done here.