Are there any moons that are made of gas?

The planets in our solar system come in two forms: Some are rocky, and some are gaseous. But all of the moons in our solar system are rocky, even the ones that orbit gas giants. So why aren't some moons in the solar system made of gas? And are there gaseous moons anywhere in the universe?

from Space.com https://ift.tt/ZwOWGKp