Sega began work on their 8-bit portable in 1989 after Nintendo's GameBoy was released. At this point it was named Project Mercury and their goal was to make a portable superior to Nintendo's. On 6th October 1990 it was released in Japan as the Game Gear. In 1991 it was released in the US and in 1992 it was released in Europe and Australia
The Game Gear is basically a portable Master System, the only major difference being a larger colour palette. It is far superior to the Nintendo GameBoy, with it's colour screen and TV tuner accessory. Another difference is that it is held lengthwise with buttons at the sides rather than the cramped format of the GameBoy. Although the Game Gear (and other portables that tried to compete with the GameBoy) seemed to be much better, the market was looking more for light weight and battery life, which the GameBoy excelled in. Game Gear uses up batteries in 3-5 hours and is rather heavy and bulky.
The likeness to the Master System meant that games could easily be ported from the Master System to the Game Gear by burning the ROMs onto Game Gear sized carts. The Master Gear converter was also released, allowing Master System games to be played directly on the Game Gear (but there was no converter that could do the reverse because of the larger colour palette of the Game Gear).
While the Game Gear never came very close to taking away the GameBoy's dominance in the handheld market, it still did better than any other portables that tried to compete with GameBoy. Sega supported the console up until 1997. In 2000, Majesco Sales resurrected it, producing a Core system at a reduced price.