Saturn has the lowest density of any planet, terrestial or gas giant.
At the standard temperature and pressure, hydrogen gas has the lowest density.
Since Pluto was downgraded to a dwarf planet, there currently is no outer planet in our solar system that is not a gas giant.
Harmful gas
Hydrogen has the lowest density of all elements. Density of it is 0.084 gm-3. It is a diatomic gas.
Jupiter is one of the four gas giant planets, the outer planets.
This gas is hydrogen.
All four outer planets, Jupiter, Saturn, Uranus, and Neptune are gas giants.
I would think that of these Jupiter would have the lowest density, since Mercury, Venus and Mars are all solid, rocky planets like Earth while Jupiter is a gas giant composed mainly of Hydrogen.
No planet - There are dwarf planets
The outer four planets, Jupiter, Saturn, Uranus, and Neptune are all Gas Giants.So Neptune is a Gas Giant.
Gas planet, giant planet or outer planet.
The inner planets are smaller than the gas giant (outer) planets.