The Galactic Gate was a device that temporarily creates a stable wormhole that allowed the Altarin'Dakor to travel between galaxies.
The gates themselves were rings of Neutronium with a central radius of 10 kilometers. This ring contained an advanced computer system that calculated its bearing and range relative to other gates to form the wormhole correctly. In addition to this navicomputer, the ring also contained a power system unlike any encountered before. The power system harnesseed the inertial forces of its own motion around nearby stars and the galactic core to create enormous amounts of energy. Placed around the interior of the ring were a dozen emitters that released high-powered laser beams (each beam has the equivalent strength of the superlaser on Deathstar II) to the center of the gate. When the beams met they formed a small ball of energy that had almost as much gravity as a singularity. At this point the navicomputer made the necessary calculations and spun the energy ball using alternating magnetic flux. This spinning motion resulted in the formation of a wormhole between two of the gates. As a side effect of wormhole travel, a ship exiting the gate was be disabled for a few seconds.
The Altarin'Dakor built two Galactic Gates, one in their galaxy and one in the main galaxy in which the New Imperium existed. They initially built the one on the NI's side first, when they initially fled during the Great War. However, that gate was destroyed behind them by the combined Alliance of Free Stars (later known as The Republic). It took the Altarin'Dakor nearly 25,000 years to completely subjugate their new galaxy and grow to the point that they felt ready to return.
Initially only scouts were sent through, but they were unable to report until a Shok'Thola who had won the right to spearhead The Return came through. Kronos won this right, and once he had entered the galaxy, he immediately took their reports and deemed it safe to invade. He then set to work building a gate on his side, which was completed sometime during the term of Chancellor Palpatine, but before the Empire was founded.
The gate was the means by which all Altarin'Dakor traveled into the galaxy. However, only Warlords and their retinues ever traveled back through the gate into the Altarin'Dakor galaxy, and then only to either meet with Altima or another Warlord, or to somehow consolidate forces or bring in reinforcements.
The Galactic Gate was the site of the climactic final battle of the New Imperium storyline, with the New Imperium, the Travelers, and several First Races all joining fores to defeat Altima and his flagship, the Infinity.