The i7-5820K does not have onboard graphics so I think this is a problem related to the motherboard supporting the Video card you have.
Perhaps other users with experience on this motherboard can share details with you. My best recommendation is for you to contact the Computer Manufacture and verify this with them.
why the motherboard isn't supporting the Video card , really this motherboard dont work with r9 270x and 280x ??????? and why its only working at gen 2 normaly
I am not sure about this because I am not a Gigabyte specialist so there could be a compatibility issue or maybe your motherboard needs a BIOS update.