"Force of nature" is an idiom. To say a person is a force of nature (is one, not has one) means the person is a very strong personality or character -- like a hurricane or a tsunami are also forces of nature -- full of energy, unstoppable, unchallengeable, unforgettable. In short, a person to be reckoned with.
Actually, gravity is the weakest of the four fundamental forces. Ordered from strongest to weakest, the forces are 1) the strong nuclear force, 2) the electromagnetic force, 3) the weak nuclear force, and 4) gravity