Gas giants are large worlds that have extremely dense atmospheres, so dense that it is impossible to land on them. It is however possible to enter the first few cloud layers of a gas giant as long as the craft is not going too fast. Currently there is no benefit to doing this, but in the future it may become a place to hide from sensors.
A gas giant can have one large moon in orbit, which is the moon equivalent to a normal planet.
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.
Harvester bay: Atmospheric Condenser Bay
Harvester bay: Particle Collector Bay