The main reason we don't get gas moons is that they are simply not big enough. Gas is very volatile, it has a high degree of movement, and this makes it easy for gas to escape. This happens on all the bodies in the solar system that have gas in the atmosphere. read more
The formation of gas giants While the Solar System’s oldest dust and gas attracted more dust and gas to form terrestrial planets, the gas giants, owing to their distance, had a head start. read more
Moons also never got big enough to become gas giants: the largest moons in our solar system are about the size of Mercury. While moons can have thin atmospheres like Earth’s or Venus’s (look at Saturn’s moon, Titan), they don’t look like mini-gas giants. read more
Most moons don’t reach this minimum size limit. As the moons are small their gravitational influence usually cannot rival that of their parent planets. This means that the planet usually strips any gas away from the moons around it. A gas moon, in the style of a gas planet could not exist. read more