Gwok Jing is probably my favorite character in wuxia fiction (definitely my favorite from the Jin Yong universe). His view of morality is probably closest to my own, and I admire the way that he worked his way from underdog status to become one of wulin's Greats. Moreover, despite all the good he accomplished, he remained to the end a humble man...never putting himself above others and always concerned about the needs of the common person.
That being said, Gwok Jing was not without his flaws. There were two choices he made, I thought, that were inconsistent with his character.
The first, which is not the topic of this discussion, is his failure to tell young Yeung Gor the truth about his father Yeung Hong early on. Gwok Jing's failure here resulted in many troubles for both Yeung Gor and himself in later years...troubles that I believe could have been prevented had Gwok Jing been more characteristically upfront with Yeung Gor about the matter from the beginning.
A greater problem, however, is Gwok Jing's continued loyalty to Genghis Khan for an extended period after he realized the Khan's ruthless ambitions.
During the Samarkand expedition, of which Gwok Jing was one of the principle Mongol commanders, Genghis ordered the slaughter of the city's civilians. Gwok Jing, naturally, was outraged by the Khan's decision to commit mass murder against unarmed civilians, but did nothing to stop the Khan. Of course, Gwok Jing was in no position to countermand the orders of the world's most powerful ruler, but the Gwok Jing I admire would have physically intervened if necessary to prevent the loss of innocent life. Instead, Gwok Jing just stood and watched as Mongolian executioners slaughtered civilians by the score.
Even more uncharacteristic, however, was that Gwok Jing remained loyal to Genghis Khan even after that incident...going so far as to invite Yau Chui Gei to the Mongolian base camp to see if the old Taoist could provide the Khan with an immortality elixir to extend his life. It took the death of Gwok Jing's mother Lee Ping (a decision caused by Genghis' attempt to maintain Gwok Jing's loyalty) for him to finally turn against the Mongols.
I can understand that Gwok Jing loved Genghis Khan like a father, and felt indebted to the Khan for caring for him and his mother and teaching him so much about being a warrior and a leader, but Gwok Jing was a character who was supposed to understand that morality trumps personal loyalties (a concept that Yeung Hong did not understand or did not accept). Despite Gwok Jing's great personal loyalty to Genghis Khan, I'm surprised that Gwok Jing did not turn against the Khan as soon as the former witnessed the mass slaughter of civilians at Samarkand. There were other clues as to Genghis' true agenda even before Samarkand, but the slaughter really should have sealed the deal for Gwok Jing. It seems out of character that it didn't.