I got a theory. Ever smell the bathroom after a fat relative was in there. Knocks me to the ground ever time. So, just imagine everything you've ever smelled before in a non way. I'd say, it just smells like gravity.
Gravity smells like the area right near a lightning strike.
We all know that a strange breed of underground monkeys run the machinery that supports the gravitational field by means of small stationary bicycles that must stay at a constant speed in order to generate enough power. Therefore, the gravity smell is a sort of melange of electricity, dirty monkey, and bike chain lube.