The Boss Gate Key, or just Boss Key, is a special type of key that appears in Sonic Unleashed and Sonic Generations. These keys are needed in order to gain access to the local level's boss, and are therefore required to be collected through the game in order to progress further.
The first Boss Keys appeared on the PS3/Xbox 360 version of Sonic Unleashed under the name Gaia Keys (ガイア神殿の鍵,Gaia shinden no kagi?, lit. "Key to the Gaia temple"). These keys were created in ancient times to unlock certain Gaia Gates (called the Boss Gate by Professor Pickle). Once unlocked, these Gaia Gates give access to the area of the local Temple of Gaia, which is guarded by the level's boss and that Sonic must defeat to enter the temple.
While there are seven Temples of Gaia in the world, only four keys need to be collected since story-wise, Sonic does not need a Gaia Gate to locate the Temple of Gaia in Eggmanland, Mazuri and Chun-nan. In Eggmanland, the Gaia Temple is located near the only stage's Goal Ring; in Mazuri, Sonic's battle with the Egg Beetle lead him to the Gaia Temple; and in Chun-nan, Sonic's search for Zonshen took him to the Gaia Temple. In the PS2/Wii version, they are replaced by Planet Tablets.
Each Gaia Key for each temple is split into to halves; one daytime half and one nighttime half, and one of each type are needed to form one Gaia Key. A nighttime key half for one continent's temple cannot be substituted with another nighttime key on another continent however, and Sonic must therefore collect each halves from the same continent in order to form the local key.
Since the two halves cannot open their Gaia Gate on their own, Sonic has to collect both halves from their local continent in order to face the boss. In order to obtain these halves, Sonic has to clear the main Stage on each continent, with the daytime half found in the main daytime Stage and the nighttime half found in the main nighttime Stage. At the end of each Stage, the Gaia Key half can then be collected, and serves as the initial Goal Ring.
In the console/PC version of Sonic Generations, the Boss Keys are needed to open the Boss Gates that are found in the White Space. Each Boss Gate found in the White Space requires three Boss Gate Keys in order to be unlocked, making a total of nine Boss Gate Keys found in the game.
After both Classic Sonic and Modern Sonic restores the colors and space of all three levels found in an area of the White Space, three Boss Gate Keys will appear in that area, but are surrounded by an unpassable barrier. In order to obtain the keys, the Sonics has to clear one Challenge Act Gate for each of the Boss Gate Keys found in their respective area. Once cleared, their barriers will be lowered and the Boss Gate Keys can be obtained. The keys will unlock the bosses Death Egg Robot, Perfect Chaos and Egg Dragoon.