I have an Asus M5A97 Evo and a FX 8320 as well
for me it shows all the core temps and runs perfectly,
it seems that it sees all the cores, but you have "only" problem with the temp readings
what does it show in the bios? only the socket temp, or the core temps as well? I do not know how Gigabyte Bios looks like, I have a fancy Asus clickBios