Gas Giants are huge planets made up entirely of gases. They are uninhabitable, and often pose extreme risk to anyone or anything entering them. Their size and relative lack of use lends them perfectly to hosting space stations around them.
Features
Unlike other planets, gas giants have no "world" within them; that is, they do not have a large plane with features on it as most other worlds do. They have very few usable resources, making them less useful to explore. As previously mentioned, they are far more useful when used to host space stations in their orbit.