Have you tried the 580 in another brand of motherboard because it would seem gigabyte bios quality is pretty abysmal.
I have a R9 290 Vapor X which has CSM and UEFI with my asus 990 fx sabertooth r2.0 and it boots fine on both UEFI and CSM whether the motherboard or gpu are set either way. Apparently this hasnt been a major case of problems for Sapphire because they didnt update the bios for your particular card. There used to be someone here by the name of
@MrGenius but he appears to have abandoned tpu that could tweak the gop portion.