As part of the Unified Community Platform project, your wiki will be migrated to the new platform in the next few weeks. Read more here.
Gas giants are non-colonizable planets, which never have moons. They are always in their own size class, giant. Superscalar gas harvester can be built on them if their system is colonized. With planetology (TL 5), a space factory can compress them into a colonizable large or huge barren planet.