You keep saying that GG is the best. He might be the best because he only fought Jedi. He might be unprepared to fight someone as good as Maul, let's face it, Obi was lucky and was just in a street-fighting mode. Against Maul, GG would need to be verrrry careful. Plus it also said in the novelization that GG basically learned all the forms by fighting Jedi. He wouldn't have seen Maul fight before and therefore would not have any advantages in a fight which gives Maul the win.