In some fanfictions, Harry could become withdrawn and depressed at first. Losing his place in Gryffindor is like losing his family. But then, he might find new strength and purpose, perhaps by joining forces with some outcasts in the wizarding world and forming a new group that challenges the unjust actions of Gryffindor 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.
He'd probably be devastated at first. Gryffindor has been his home at Hogwarts, and his friends are there. But he's also a resilient character. He might start to question the fairness of the banishment and try to prove his innocence. He could look for allies outside of Gryffindor, like Luna from Ravenclaw, who might believe in him.
In some fanfictions, it could be a political move within Hogwarts. There might be a group that wants to undermine Dumbledore's influence, and since Harry is so closely associated with him, they target Harry by getting him banished from Gryffindor. This could also be related to the discovery of some long - hidden secret about Harry's family that makes the Gryffindors view him differently.
Another reason could be related to a new prophecy. Suppose a new prophecy emerged that said Harry Potter's presence in Gryffindor would bring great danger to the house. The Gryffindor students and faculty, believing in the importance of protecting the whole house, might have decided to banish him. But of course, this would be a very difficult decision as Harry has been an important part of Gryffindor before. And in the end, the prophecy might turn out to be misinterpreted.
He might become more independent. Without the support of Gryffindor, he has to rely solely on himself. So he becomes more self - reliant and his decision - making skills improve.
There could be many reasons. Maybe some fanfiction writers want to create an alternate universe where Harry has a different path. It could also be for the sake of creating a more complex and unexpected plot, like having him be falsely accused of something so bad that Gryffindor doesn't want him. Another possibility is that the writer wants to explore how Harry would react and grow outside of the Gryffindor environment he's so closely associated with in the original series.
In Gryffindor fanfiction, Harry getting 150 points could be a significant plot point. It might be a reward for his heroic deeds, like saving a fellow student from danger or uncovering a mystery that benefits the whole school. This sudden addition of points could boost Gryffindor's standing in the House Cup competition and also enhance Harry's reputation among his peers. It could lead to a celebration within Gryffindor common room, with everyone cheering for Harry and feeling proud of their House's lead in the points race.
Well, Gryffindor could protect Harry by being there for him emotionally. In the fanfiction, Gryffindors might offer him a safe place to talk and share his troubles. They would encourage him when he feels down or doubts himself. Also, Gryffindor as a whole House could provide Harry with resources, like access to certain magical knowledge or artifacts within the Gryffindor common room that can help him in his battles against the dark forces.