The Sega Gear-to-Gear Cable was a link cable used to connect one Game Gear to another for multiplayer games. Both systems need a copy of the same game, as the cable is only used for communication between the systems.
The Gear-to-Gear port can also be used with a Master Link to add a second controller for Master System games played using a Master Gear Converter.
Click here for technical information about the Gear-to-Gear Cable.