Gon the happy go lucky boy,a child naive in the sense of the world, developing and discovering his own personality through a great adventure by going out, he unlike Killua who had a great experience and understands the world, Gon only relied on his emotions which people consider a weakness, he is a child, who grew up on a little island in the middle of nowhere with no friends his own age. We expect him not to throw a tantrum when confronted with such a stark contrast of what was expected, and what turned out to be real? Him acting rational to me would be a betrayal of the facts that surround his life. In contrast Killua and Kurapika had their own dark moments which have made them quite mature for their age.
The thing is about Gon is that he's just a little kid. And the entire story up till this point is really just about this kid leaving his home and adventuring out into the real world and seeing how **** up a place it is, mainly by meeting people who kill for no reason and seeing how that conflicts with his ideals. Gon doesn't really have a feeling for evil or good, he is just a being of pure curiosity (like how zepile said in yorknew). This whole story is just about how this kid ventures out into the world and reacting to the events that happen to him. His character is supposed to be ambiguous on purpose because he is the protagonist of this adventure story and it is driven by his character and the character of others. The reason why we haven't seen gon progress is because this we are supposed to learn of his character through interactions he has within the story itself (like an adventure manga should be). He isn't like Killua with a history he has to overcome he just goes on adventures and interact with people he's never seen in his life and overcome the conflicts that he is faced with like this conflict with Pitou which is one of the most complex he's ever been in. Pitou is the only person Gon has actually hated and despised. Remember in the hunter exams when hanzo said he gave up fighting gon bcuz he had no hate in his eyes despite breaking his arms. Well for the first time he actually had legitimate hate for Pitou bcuz he killed Kite, the most important person in the entire world to him next to his aunt mito. He doesn't give a damn about Ging if it wasn't for Kite then he would have never went looking for him, if it wasn't for kite he would have never been a hunter. Kite is literally the reason for everything in this manga happening and is the most important person to Gon so when he died he was angry and wanted vengeance against the monster pitou. But the irony is was that that monster was healing a human when it was thought that they were incapable of caring for humans and were hell bent on making them extinct. Its especially maddening for Gon because of what he saw Pitou do to Kite the man he has the most respect for in the entire world. It makes sense for him to get mad at Pitou for his hypocrisy, hell pitou doesn't even really care about komugi he's just trying to figure out how to survive long enough to kill Gon and Killua. This situation is way more complex than oh Gon is just a mad spoiled brat. No he deserves to be mad and he should have not hesitated to fight Pitou then and there bcuz its their mission.
At least Gon can show emotion like a real human being and not unrealistically understanding and nice to a psychopathic monster trying to kill you the first chance he gets like some other generic shonen.
In each and every arc he has developed little by little and the blank book is being filled through experiences.
And Gon is one of the weaker characters in terms of development, others like Killua, Kurapika, Hisoka are much more developed and complex.