The Sega Master System and Sega Game Gear are interchangeable. The SGG was designed as a portable SMS. With an adapter, you can play a SGG on the SMS and vice versa (except for games reliant on Cards). They're essentially the same hardware. So designing an emulator to emulate the SGG or SMS, it will also play games for the other. You would have to deliberately block specific games to make your emulator not emulate SGG or SMS games.
Close. The Game Gear is more advanced than the SMS (I think the biggest thing is it has more colors), and while there are adapters to play SMS carts on a Game Gear, in general there is no way to play GG games on an SMS. Most emulators are made with this in mind because it's really not harder to implement the GG specs and have both rather than just the base SMS specs and only have that.