Actually, when I think about that, it just shows me even more poignantly how inept the 7 Freaks were as teachers.
GJ still managed to become a better-than expert archer under the tutelage of Jeibei. He also mastered Mongolian wrestling. It seems that with proper guidance (or at least not having 7 teachers teaching at least 7 different skills at the same time), GJ would've done well in martial arts even if he did not achieve Great status.