Kenseiden was never released for the Game Gear. That's a SEGA Master System pirate on a Game Gear Cartridge; even more evident by the fact that the SMS-style "white boxes" cover border is still there, while Game Gear games NEVER used this (though some games used GREY squares on the boxart in Europe.)

At the very least, it's a much better made pirate than most, but the actual value is incredibly hard to place.