Another possibility is that in this fanfiction, Gryffindor's personality is depicted in a way that clashes with Harry's. Gryffindor could be shown as arrogant or self - centered, and Harry, who values humility and selflessness, just can't overlook these flaws and thus doesn't forgive him.
Maybe he feels betrayed by Gryffindor's actions or values that have been misrepresented in the fanfiction. It could be that Gryffindor did something that went against Harry's own moral code, like sacrificing the innocent for the so - called greater good.
One possible reason could be that he feels abandoned. His parents died when he was a baby and he grew up without them. He might think that if they were more careful or made different choices, they could have been there for him. For example, if they hadn't gotten involved in the fight against Voldemort in a certain way, they might have survived. Another reason could be that he has heard so many stories about them but has no real memories of them. It's hard to forgive someone you don't really know but are supposed to have a deep connection with just based on what others say.
Maybe Gryffindor did something that seriously violated Harry's principles. For example, he might have endangered Harry's friends or made a decision that put Harry in great danger without his consent.
It could also be that his siblings continuously rejected him and his ideals in the past. Naruto has always strived for peace and acceptance, and if his siblings constantly opposed him on these fundamental things, it might lead to his unwillingness to forgive.
One possible reason could be years of neglect. Maybe his family was absent during his difficult childhood in Konoha, leaving him to fend for himself. He had to endure loneliness and the scorn of the villagers without any family support, which could lead to deep - seated resentment and an inability to forgive.
Well, perhaps in this fanfiction, Kushina did something that violated Naruto's principles. Naruto has always been about loyalty and hard work. If Kushina did something like betraying the village or his friends, Naruto, being the stubborn and principled person he is, might not forgive her. Also, it could be that Kushina was absent during some crucial moments in Naruto's life and he holds a grudge for that.
One possible reason could be a fallout with his friends in Gryffindor. Maybe he feels betrayed by them in some way in the fanfiction world. For example, if they wrongly suspect him of something.
Perhaps he leaves Gryffindor in fanfiction because he is influenced by a new mentor or friend who is from a different house or group. This person might convince him that he can achieve more or be more true to himself outside of Gryffindor. Also, it could be due to a prophecy or a special mission that requires him to distance himself from Gryffindor for a while to protect the house or the entire Hogwarts from a certain threat.
The relationship between Harry and the Gryffindor house is likely to be strained. Since Gryffindor is the namesake of the house Harry belongs to, this lack of forgiveness could cause a rift among the students. Some might side with Harry, while others might defend Gryffindor's legacy, creating internal conflicts within the house.
In some fanfictions, it could be due to a misunderstanding. Maybe someone framed Harry for a serious rule violation that Gryffindor's house values highly, like cheating in an important magical exam. And the evidence against him seems so strong that the Gryffindor house decides to banish him.
One possible reason could be a false accusation of using dark magic. In some fanfictions, the wizarding world can be very paranoid about dark magic, and if Harry was wrongly suspected of delving into it, Gryffindor might exile him to protect their reputation.