Gas Giant
Gas giants are large worlds that have extremely dense atmospheres and very strong gravity. The atmosphere is so dense that it is liquefied and forms a incredibly deep ocean world. It is possible to dive into the ocean of liquefied gasses with a submarine or a submersible spacecraft.
Harvestable Atmospheric Resources
Gas giants are normally rich in various atmospheric resources which can be harvested using harvester bay equipped spacecraft. This is normally also very advantageous as the high atmosphere density means greater harvest yields.
The resources in the atmosphere depends on the orbit zone of the gas giant. A gas giant in the inferno orbit zone might have ioplasma, and a gas giant in the fridge zone might have cryozine.
Atmospheric Resources
Harvester bay: Atmospheric Condenser Bay
Particle Resources
Harvester bay: Particle Collector Bay
Colonization
At the center of a gas giant is a rocky core. The rocky core can be colonized with buildings and and cities just like normal worlds.
Construction is a harsh environment, but it also requires grav couplings to counteract the strong gravity.
Core Resources
The rocky core has the most of the same resources as regular worlds. There are however a few unique resources that are not found anywhere else.
Life
A gas giant with an Exotic atmosphere might have strange underwater creatures in its ocean.