My GSkill 2400 needed 1.65V to do its thing at XMP on X79 (this is stock, though). Try a little higher on the voltage?
I always found X79 notoriously picky with RAM speeds - moreso on ASUS boards, particularly the Rampage ones - XMP was quite spotty for me. I had a lot of boards, loved X79. I can't speak for your Gigabyte one though, sorry.
I'm (back) on Z77 now, and can get the same timings at 1T at 1.6V. A bit tighter.
7700K, 32GB, 980Ti, 960 EVO, PG278Q - DG-87