The fanfic might be full of clichés about Gryffindor that Harry has grown tired of. For example, always portraying Gryffindors as overly brave without any real depth or complexity.
One reason could be that the fanfic distorts the characters he knows well from Gryffindor. For instance, if it changes the personalities of his friends in Gryffindor in a way that doesn't seem right to him.
It's possible that the Gryffindor fanfic he has read contains a lot of OOC (Out Of Character) situations for the Gryffindor characters. Harry, who knows these characters intimately, might find it really irritating when they are made to act in ways that are completely different from their true selves in the fanfics.
One reason could be that he doesn't like the spotlight that comes with being a Gryffindor. In the original story, Gryffindors are often in the limelight, and in fanfiction, he might be tired of all the attention.
One reason could be a personal betrayal. Maybe a close friend from Gryffindor betrays Harry, and this makes him start to dislike the whole house. It's like when one bad apple spoils the bunch in his eyes.
Perhaps Harry has his own vision of Gryffindor and the fanfics he's seen don't align with it. He might have a deep - seated understanding of what Gryffindor means to him, like courage, loyalty, and friendship in a very personal way. The fanfics, on the other hand, might focus on more superficial or inappropriate aspects of Gryffindor for his taste, such as focusing too much on power plays or false bravado rather than the true essence of what makes Gryffindor great.
One reason could be a falling - out with his friends in Gryffindor. Maybe there was a misunderstanding that grew so large that he felt he couldn't stay. Another possibility is that he might be lured away by some mysterious force or character who convinces him that Gryffindor is no longer the right place for him. For example, if a new group claims to have a better way to fight Voldemort and they are from a different 'house' in this fanfiction concept.
It could also be that the fanfic might interfere with the way he wants his story to be remembered. His adventures at Hogwarts were real and important to him, and when fanfic changes the narrative, it's like his story is being disrespected. For example, if a fanfic portrays him as cowardly when he was actually brave, that would surely make him dislike it.
In some fanfics, Harry might be made to hate Gryffindor perhaps because the author wants to explore an alternative personality for him. Maybe they want to show how the Gryffindor house's values, like bravado or recklessness, could be seen in a negative light from Harry's perspective.
One possible reason could be a major disagreement with the Gryffindor house leadership or some of the students. Maybe he feels that they are not living up to the values he thought they held. Another reason might be that he discovers some dark secret within Gryffindor that makes him uncomfortable and he decides to leave.
Perhaps in this fanfic, Harry has been influenced by dark magic or some evil entity. This external influence could distort his perception of Lily and turn his normal feelings for her into hatred. Dark magic has the power to corrupt one's mind and change relationships in unexpected ways.
Another possibility is that he doesn't like how some fanfictions over - sexualize the characters. In some cases, fanfiction can turn into something inappropriate, and Harry, who has a more pure view of his friends and the wizarding world in general, would be disgusted by this. He wants to preserve the integrity of the relationships and the world as he knows it.
Maybe the fanfic often misrepresents the nature or relationship of Bellatrix and Rodolphus. For example, if in the original they are portrayed as very dark and loyal to a certain cause, but fanfic makes them out to be something else entirely, like soft or good - hearted, it could lead to dislike. Also, some people who are die - hard fans of the official story just don't like the idea of others taking creative liberties with their beloved characters.