In GT racing you have different classes that you can enter your car into... GT 1, 2, 3 or 4. Each class reflects what spec of car you can run, e.g. GT1,s may have 500hp and GT4's may only run 300hp abiding to the regulations (they arn't exact figures). So... GT3 isnt z4 specific, you can have a 911 GT3 like in the vid, its just the class that it runs in.