The Galactic Gate is a non-buildable unit appearing only in campaign mode; it was used by both the CORE and the ARM during the war to transport Commanders from planet to planet, and played a vital role in the near-destruction of the player's faction and the eventual downfall of the enemy faction in the respective campaigns.
The Galactic Gates were built to teleport units from planet to planet, but the amount of resources required to open the gate was monumental, thus only important units like a Commander or Construction unit were sent through to conquer enemy planets.
It is never revealed how an ARM Galactic Gate was built on CORE Prime, as the amount of time that it would have taken to build the gate would far exceed the time it would take for the CORE to notice it was being built, providing the CORE's defense systems didn't prevent construction from taking place to start with; the only logical explanation would be that the ARM built the gate on another planet and transported it to CORE Prime via unknown means. After finding out about the Galactic Gate, the CORE Commander hatched a plan to capture said gate and use it against the ARM to wipe the galaxy of their presence. As the CORE eradicated the ARM forces on a planet, the Commander would capture their galactic gate to move onto the next planet in an attempt to reach Empyrrean.
While the Commander was being lured to the ARM outpost of Calabran, the CORE snuck on to the surface of Empyrrean to build a Galactic Gate; in an attempt to return the ARM Commander back to Empyrrean, a small group of ARM units made a last ditch attempt to push past the CORE units that were stopping them from reaching their own gate. The group were successful in returning the Commander home, and initiated the turning point that would lead the ARM to Victory. After purging the CORE forces on a planet, the ARM Commander would capture the CORE Galactic Gate and use them to teleport to the next CORE infested planet and ultimately the CORE's homeworld, CORE Prime.
The ARM Galactic Gate is somewhat stronger and takes longer to build than the CORE Galactic Gate, this is unusual for the ARM, as they usually prefer units with lower build times but lower armour. The better armour of the ARM Galactic Gate may be because the ARM are more reliant on them than the CORE, as the seem CORE have the ability to send units through space via other means.
Destroying a Galactic Gate that is meant to be captured during a mission will result in defeat for the player.