From Sega Retro
The Wide Gear, known as the Big Window (ビッグウインドー) in Japan, is the original screen magnifier for the Sega Game Gear. The Game Gear received more than its fair share of magnifiers, several of which were officially licensed, but this is likely to be the first and most simple iteration of the device.
The Wide Gear does as the name suggests - it makes the screen "wider". It is merely a transparent piece of plastic mounted to the Game Gear unit at the bottom which acts as a lens and enlarges the picture for easier viewing. The Wide Gear is just plastic - it does not increase the resolution of the display nor the quality of the Game Gear's built-in screen.
The Wide Gear is not a flexible accessory - it has to be physically screwed into the Game Gear and will always hover above the screen at a certain distance. It cannot be folded nor is it particularly strong, and as it is not simple to install, the device is not considered to be very "portable". Working Wide Gears are hence not very common - Sega would roll out a replacement line of Super Wide Gears instead, which would become the official screen magnifier of choice.
The Wide Gear does not appear to have been released in Japan officially, though a variant by Sunsoft exists.