He could use it as motivation to improve. Maybe there are aspects of the novel that he can work on, like the marketing or the plot development. He could also try to engage with the people who made the comment in a polite way to understand their perspective better.
Sasuke would probably be initially skeptical. He might think that Naruto is relying too much on an external weapon instead of his own natural abilities. However, he would also be intrigued and might keep an eye on Naruto to see how he uses it. Sakura would be worried. She would constantly remind Naruto about the dangers of the gun and how it's not a traditional ninja weapon. She might even try to convince him to get rid of it.
I think Naruto's friends would be quite shocked. For the more traditional ninja like Neji (if he were still alive in this fanfic), he would see the gun as an abomination to the ninja way. However, Naruto's closest friends like Kiba would be more open - minded. Kiba might see it as a cool new weapon and want to see what it can do. But overall, they would all be concerned about how Naruto got the gun and what it means for their world of ninjas.
The characters' reactions would be diverse. Snape would likely be very critical and use it as another reason to dislike Harry. He might see it as a sign of Harry's lack of respect for the wizarding world's traditions. The other students would be a mix of those who are fascinated by this strange Muggle object and those who are worried about its potential for harm. Hagrid would be confused, not really understanding what a gun is but knowing it doesn't seem right in Hogwarts. Overall, it would create a stir among the Hogwarts community and lead to a lot of discussions and speculations.
Natsu's friends would have a range of reactions. Lucy would be distraught, her face pale with fear as she kneels beside Natsu. Gray would clench his fists, his eyes filled with rage as he scans the area for the shooter. Erza would be the voice of calm and reason, though her hands would be shaking slightly as she tries to stop the bleeding. Happy would be flying around in a frenzy, screeching in distress. They would all be united in their determination to save Natsu and seek justice for what has happened.
Hermione would likely be shocked and disappointed. She is a stickler for the rules and the gun would be a blatant violation. Ron might be initially curious but then realize the seriousness of it. Dumbledore would be very concerned and try to understand Harry's motives. Snape would probably be quick to condemn Harry and use it as an excuse to give him detention.
Daniel was a figure known for his unwavering loyalty to God. He served in a foreign court but still adhered to his religious beliefs. This made him some enemies who plotted against him and got him thrown into the lions' den.