Gas Giants are a later game objective. Using Gas Giant Platforms, you can colonize a gas giant once your University gets to level 19.  Before attempting to colonize a Gas Giant be sure to read up on how to deploy and what strategy to use to successfully colonize them.  Gas Giants range in size from 70 to 121.  Since you'll need at least four platforms at level 30 to utilize the largest of the Gas Giants, 117 is the largest size one could use.

Gas Giants and their mineral compositon can be found at the Body Types page.

Some colonization strategies:

• Use a complete set of basic resource glyph buildings such as the Natural Spring, Volcano, Algae Pond, and Malcud Field
• Use the storage glyph buildings Interdimensional Rift and Ravine
• Do Not use high level platforms as your intial plaforms when colonizing a Gas Giant.  Stick to level 4 or 5 platforms.  High level platforms produce large amounts of trash that a fledgeling colony will not be able to handle.  A level 10 produces about 5000 trash per hour.
• Trash is going to be your biggest concern, so come up with a plan to deal with it using only a few plots.  One strategy that works well is to use a Trade Ministry, Shipyard, Ravine, Interdimensional Rift, and a Cargo Ship.  Then ship all of the trash off world until you can get the proper facilities built to deal with it on world.  Scows can also be used, but be sure to have one with a low storage capacity since a Scow can not be launched until it's hull is completely filled. 
• When looking for Gas Giants to colonize ignore any Gas Giants that are less than 90 plots in size unless you really need them for a specific reason.
• Gas Giants have no natural water so Atmospheric Condensers do not work on them.
• Gas Giants which produce Sulfur are also handy as this is a needed resource for the Water Reclamation Facility which will likely be your primary source of water.
• Be very wary of using Gas Giant Platforms in excess of level 15.  The amount of trash produced by these high level platforms again would be difficult to handle unless the Gas Giant is a well developed Capital Planet.