Gravity is a magnetic force, therefore, it would have a smattering of a metal or fine nickle smell to it. After all, its been around a long time. Unfortunately, humanity will never know what gravity smells like. The odor of gravity is identical to the odor of air, but since there's so much more air than gravity you can't even catch a whiff of the ultimate downer. We can tell that air is more common than gravity because you can get away from gravity in an airplane.
Air is everywhere, man.