The Boss Gate first appeared in the PlayStation 3/Xbox 360 version of Sonic Unleashed. These portals are specialized Gaia Gates created thousands of years ago to give someone access to the nearby vicinity of the local Temple of Gaia. In this game, the Boss Gates are circular pads with a red light emerging from them which displays the symbol typically associated with Dark Gaia. They are named Boss Gates by Professor Pickle due to the strong opponents that guard the Temples of Gaia.
To unlock a Boss Gate in most cases, the player has to obtain the Boss Gate's Gaia Key, by collecting its two halves from each continent in order to activate the Boss Gate. These halves are collected by completing the continent's night and daytime Stages.
In the PlayStation 2/Nintendo Wii version of Sonic Unleashed, Boss Gates are not featured; instead, there is a door with star patterns on it inside each Gaia Gate that leads to the Boss Stage. The player can unlock these doors by assembling the matching Planet Tablet from the fragments collected in the Stages.
In Sonic Generations all of the Boss Gates are located in White Space. The outer layout of the Boss Gates is a floating portal made of gears, cogs, and locks. Just like the other places found in White Space, the Boss Gate leads to a different point in Sonic's history, but instead of just a location, it brings Sonic into the middle of battles with some of his biggest opponents (The Death Egg Robot, Perfect Chaos and the Egg Dragoon). In order to open a Boss Gate, the Sonics have to collect three Boss Gate Keys for each Boss Gate, which is scattered throughout the White Space. After clearing a Boss Gate, a Chaos Emerald is restored from its colorless state and passed on to one of the Sonics.
In the 3DS version, the boss gates have both a rival and a big opponent (Metal Sonic and Big Arm in the first one, Shadow and the Biolizard in the second one, and Silver and the Egg Emperor in the third one). In order to open the gates, the Sonics must restore all of the zones of each era. Once they reach a boss gate, they must defeat the rival in a race before they can battle the major opponent.
At the far end of White Space is the Time Eater's boss gate, which is much larger than the other ones and can only be activated by the Chaos Emeralds.
Unlike the other areas in the White World, which leads directly to a point in Sonic's history, the Boss Gate's entrance first leads to the interior of the gate, which features a rather large black and white hallway, decorated with white chandeliers and (at least in the Death Egg Robot's Boss Gate's case) a wall with transparent glass cutting through the middle of the room in which each end of the hallway has a stone door that leads respectively to the boss and back to White Space.