Try enabling it and see if the resource monitor shows 8 cores.
Gigabyte has made boards before that have enabled disabled features. I have a GA-MA790XT-UD4P (I'm pretty sure that is the model) and an AMD 720BE Phenom II X3 that the motherboard successfully made into a 4 core processor. So re-enabling disabled features is sometimes possible.
That said, if it were possible for your circumstance, somebody else likely would have discovered it long ago, since your processor/motherboard combo has been around a while.