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.
His friends from Team 7, like Kakashi, would feel responsible for Naruto's situation. Kakashi would likely use his resources to find a proper shelter for Naruto. Shikamaru and the others might come together to form a sort of support group for Naruto. They would pool their resources, like money and food, and also try to cheer him up by sharing stories and jokes.
Sasuke, who has his own complex relationship with Naruto, would probably be in denial at first. He's seen Naruto as this strong - willed person, and the idea of him having schizophrenia would be hard to accept. But as time goes on, he would become more protective. He might train Naruto in ways to control his mind during the episodes, using his own experiences of mental discipline. As for Kakashi, he would be empathetic. He has his own traumas, and he would do his best to support Naruto, maybe sharing his own stories of dealing with difficult mental states.
Sakura might be initially shocked and heartbroken. She has been Naruto's friend for a long time, and his sudden departure would leave a big hole in her life. She might try to convince him to stay at first.
Hinata would be heartbroken. She has always loved Naruto deeply. She would probably be in a state of shock at first and then mourn for a long time. She might also try to keep Naruto's legacy alive, perhaps by teaching his ideals to the next generation of ninja.
Sakura would probably be a bit skeptical at first. She might worry that Naruto is getting distracted from his ninja duties. But once she sees how passionate he is, she would support him and might even help him with his training regime, using her medical knowledge to make sure he's in top physical condition for racing.
Sasuke would be extremely enraged. He has a deep bond with Naruto, and Naruto's death would likely send him on a quest for revenge against Konoha. He might see it as a betrayal of everything Naruto stood for and fought for.
Shikamaru would probably be the most vocal against it. He's a lazy but intelligent character. He would see Naruto's plan as too troublesome and dangerous. He'd tell Naruto that there are too many unknowns and that it's not worth the risk. He might even try to enlist the help of Kakashi or other more experienced ninjas to stop Naruto from going through with it.
His friends would be very shocked at first. Sasuke, for example, might be suspicious of this new Naruto. But once he realizes that Naruto is still the same at heart, he would start to accept it. Sakura would probably be worried about Naruto's well - being and try to find a way to reverse the change if possible.
Sakura might be a bit confused at first. She's focused on her medical ninja training and Naruto's sudden interest in Lunney seems out of left field. But she would probably be supportive in the end, especially if Naruto could show her how Lunney's ideas could be relevant to their lives. Sasuke, on the other hand, would likely be dismissive. He has his own goals and Naruto's newfangled interest might seem like a distraction to him. However, if Naruto could prove that Lunney's concepts could help in their fight against common enemies, Sasuke might change his tune.