"To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, become the Seat of the Government of the United States".
Since Congress has not exercised its EXCLUSIVE power to pass legislation on this topic, Mr. Gregory has not violated any binding law.
I think SCOTUS disagrees with you about the power of the DC gun laws. If they had no teeth as you suggest, why did the SCOTUS overturn them? Heller would not have had standing to sue.