Gongzi is what NiF an The Untamed both translate as "young master" - a son of a noble/distinguished house who doesn't have another title, like Jingui, Yujin, and Lin Chen (whose father is the Old Master). It seems reasonable enough. I've also seen "childe" in the Buffy sense, but in this case they were probably going for a misplaced archaism like "Childe Roland to the dark tower came" and missed the tone.