Actually, “Who Goes There?” was set in Antarctica, near the magnetic pole. The Thing wouldn’t have been nearly as much of a threat on some other planet; the fear was that it would take over the Earth. The most prominent surface-of-Jupiter story I can recall would be Asimov’s “Not Final!”, and its sequel “Victory Unintentional”.
And I should point out that to determine the density of the planets, you also need to know their masses. Fortunately, though, the gas giants all have moons, which makes it easy to determine their masses.