Cheung Ah Sang, Gong Nam Freak # 5, perished before he could teach Gwok Jing a single stroke of martial arts. Nevertheless, in some ways, this teacher might have influenced the very young Gwok Jing (age six) as much or more than his subsequent teachers did.
Cheung Ah Sang sacrificed his life to protect his brothers/sister and Gwok Jing from Mui Chiu Fung and Chan Yeun Fung. From this, I think the young Gwok Jing gained the lesson that sometimes, a hero must sacrifice himself for the greater good. This idea seemed to underlie many of Gwok Jing's motives later on during his heroic career, including his final decision to sacrifice himself and his family protecting Seung Yeung from the Mongols decades later.