Gas Giant Settlement Ships deploy into Gas Giant Settlement Platforms, which allow you to colonize a gas giant. It sets up a gravimetric shield which counter acts the massive force of the giant's normal gravity, and begins to form a solid surface from which to build a colony. You need a Gas Giant Lab to build these ships.
The level of the platform created by the ship is determined by the level of the gas giant lab on the planet that launched the ship. For example, if you launch a gas giant settlement ship from a planet without a gas giant lab, then the platform plan you get on the planet will be level 1. But if you have a level 3 lab, then the plan will be 1+3, resulting in a level 4 platform.
Note: You have to have University at Level 19 or above to be able to use a settlement ship.