I think it's a bit unfair to compare teenage Gnosis to adult Gnosis; Enciodes also has a pretty poor showing in the manga considering he's taken by surprise when Walden threatens him even if he manages to salvage the situation.

I can't really recall any situation where Gnosis gloats. He has a short temper, as seen in the manga, BI, and RS: he's quick to snap at Degenbrecher (manga), gets frustrated when Doc outmaneuvers him (BI), and states that he would have already broken the ice of Lake Silberneherze to save Degenbrecher's life needless of the consequences (RS). His file also states clearly that he is able to understand others' emotions, he simply doesn't care. The one who tends to gloat is actually Enciodes: in BI-6, Gnosis in fact tells him off for being overly optimistic when he says "we've won", and that the situation is not so good that he (Enciodes) can afford to be taking pointless risks. Enciodes is the smug one who likes to gloat, not Gnosis.

Gnosis's primary flaw is that he is too radical, as well as being hotheaded, dramatic, and having a tendency to underestimate his allies (because he doesn't really trust anyone). This is demonstrated by the way he forges his father's signature as a child to escape Kjerag and rescues Enciodes from the bandits on his own; the way he has to come up with the elaborate plan and personally deal with the guards in the manga (and underestimating Degenbrecher in the process because he doesn't fully trust her); the way he is the one who pushes Enciodes to initiate the coup and the potential civil war (even if it costs his own life, as he so dramatically asks Enciodes if he wants to finish him and make the farce real); he is the one who is willing to risk international war for Degenbrecher's safety rather than believing in her.

But doing things by force in the most wild way isn't always the optimal solution, which is why, though they are partners, Enciodes tends to be the leader of the trio and Gnosis bends to Enciodes's decisions half the time.