Gravity is not a law - it is a theory. Newton's theory of gravity is incorrect (it fails when the gravitational field gets too large). Einstein's theory of gravity is also wrong - it also fails in certain instances (it predicts an infinte gravitational field strength at the very center of a black hole). Any theory which has an infinity in it is wrong. It may work well for most situations, but it is only that a theory. It's fairly easy to see when Newton's theory fails: F = GMm/r^2 - when r -> 0, the field strength becomes infinite. If either theory were a "law" both would be correct and a lot of physicists would be/have been out of business, say like Einstein.