In planetary classification, a class 6 gas giant was a type of gas giant planet. They might be hard to distinguish visually from Class 7 gas giants. (ENT: "Broken Bow")
In 2375, the crew of the USS Voyager ordered their new multi-spatial probe deep into the atmosphere of a class-6 gas giant, to prevent it from being stolen by the Malon. The probe became lodged in a layer of liquid hydrogen and methane, and was later retrieved by the Delta Flyer. (VOY: "Extreme Risk")