re: NASA's Juno craft beams back stunning pics of Jupiter.

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There could be gases underneath the atmosphere that we cannot detect or wouldn't know about due to our tech.

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Any gas we encounter will still be just isotopes of hydrogen and helium etc. There are no undiscovered light elements and we will never discover another one. These elements fall on the periodic scale based on how many protons it has. Hydrogen has one proton and uranium has much more for example. Everything we encounter with one proton is still hydrogen. We already know how any undiscovered element will behave and it will be heavy and only live a fraction of a second.

We know what makes up the planet by analyzing its spectrum and other high tech devices at our disposal. The Galileo mission in the 90's dropped a titanium probe into the atmosphere giving us firsthand knowledge of its contents. We know its mass by looking at the orbit of its moons and how fast they orbit. Closer to a likely solid core the hydrogen and helium are in a liquid like state due to pressure.