While we have observed the gravitational constant to be just that- a constant- and it has largely been proven to be true for celestial bodies, it's worthwhile to note that we haven't proven it's always true anywhere in the universe. Nor can we postulate it through logic because honestly we have NO knowledge of how or why gravity happens. It just does. In fact it's generally regarded as the least understood of the known fundamental forces of physics. While electricity and magnetism are inherently tied together as compliments, we have no such compliment to gravity. More puzzling, there is + and - electric fields but so far we've never seen a negative gravity effect. Is there a negative gravity effect? Antigravity? Well a lot of things familiar, exotic, and mind-bogglingly complicated have been through the physics labs of the last hundred years and it hasn't been seen in any degree, but this does not constitute proof that such a thing does not exist. Yet I'd like to add that it doesn't mean that creating such a fundamental force of nature with spinning tops and magnets is plausible either.