Although it would take a lot of energy to escape the gravity well of a gas giant, all of the gas giants are also far away from the sun. In the case of Uranus,

Although it would take a lot of energy to escape the gravity well of a gas giant, all of the gas giants are also far away from the sun. In the case of Uranus, this means that there is more energy to be gained from dropping towards the Sun to Earth than would be lost by escaping Uranus' gravity well. For a visual representation of this, see https://xkcd.com/681/ https://xkcd.com/681/ So, one could imagine a siphon which would take helium from Uranus to Earth. A normal siphon moves a liquid out of a gravity well by relying on a deeper gravity well. Of course, there a limits to the height that siphons can handle, based upon the vapor pressure of the fluid, and the idea of running a really long tube from one planet to another is kind of ridiculous. However, the same energy differences could be used in principle to power a more realistic scale system, like a recirculating ship which gains kinetic energy on the Uranus-Earth run and loses it on the pick-up-helium-at-Uranus run. Such a system would need to dump angular momentum around the Sun at one of the planets, because this doesn't balance when you move mass from one planet to another. My inner five-year-old loves the idea of mining gas from Uranus. :)