The Gear Master is an accessory for the Sega Game Gear and an alternative to the Master Gear Converter. This one was manufactured by Beeshu and presumably sold with a license from Sega.
Like the Master Gear Converter this peripheral allows Sega Master System games to be played on a Game Gear, the major difference being that it does not give you the means to screw the device into the Game Gear unit. It is presumed to have the same (or similar) compatibility issues as the Master Gear Converter. It also did not likely leave North America.