The Star Gate acts as a "Gate" into other star systems to explore. The three systems are the Dwarf Star System, the Pulsar System and the Red Giant System. Also there is a Star Fortresses that goes along with their corresponding Star System.
The Star Gate is the only building in the game you don't need to build.
This is just for iOS as of 12/09/2013.
As of 03/03/2014 Andriod has the Star Gate