Perhaps an old enemy who has infiltrated Gryffindor in disguise. They use their influence to make the Gryffindors believe that Harry is a danger. They could be a Death Eater or a follower of Voldemort who has found a way to get into Gryffindor and manipulate the students' minds, causing them to betray Harry.
A dark force that has the ability to influence the minds of Gryffindor members. This could be a powerful and ancient magic or a new form of dark magic. It turns the house against Harry without them fully realizing it, making the collective Gryffindor house, under this influence, seem like the antagonist.
If Harry's parents are the ones who betrayed him, they could be considered the main antagonists in a sense. They created this false image of themselves as loving parents when in fact they had ulterior motives. Another possibility could be a character who was in on the parents' plan and now is trying to cover it up or continue the malicious work they started. Maybe it's an old acquaintance of his parents from their days at Hogwarts who is now out to stop Harry from finding out the truth.
The Ministry of Magic itself could be the main antagonist. They might be so afraid of Harry's growing power and influence that they create false evidence to make everyone believe he has betrayed the wizarding world. They try to control the situation by turning everyone against Harry.
Perhaps an ancient evil that Gryffindor once sealed away is starting to break free. This entity sees the descendant as a threat because they have the potential to reseal it. So, it tries to corrupt the descendant or stop them in any way possible.
In a 'Harry Potter betrayed by Gryffindor' fanfiction, perhaps there is a prophecy that is misinterpreted. The Gryffindors think Harry is the cause of upcoming doom instead of the savior. So, they begin to betray him. Harry would then have to struggle not only with the external dark forces but also with the internal pain of being rejected by those he considered friends and housemates. He might start to question his own identity and values, and his journey would be about regaining the trust of Gryffindor or finding his own path without them.
In some cases, it could be an ancient and evil magical being that has been awakened. This being has the ability to manipulate people's minds and convinces everyone that Harry is evil. It wants to isolate Harry before attacking him directly for some unknown but dark reason.
Maybe it's a new Gryffindor student who has been influenced by the wrong people. This new student doesn't really know Harry well and is easily convinced by some anti - Harry propaganda within the school. So they become the main antagonist in the story and lead the attack on Harry.
It could be a new player on the team who is jealous of Harry's skills and popularity. This new player might spread lies about Harry, turning the other team members against him. So the new player would be the main antagonist in this scenario.
It could be a power - hungry Slytherin who doesn't like that Regulus is showing an interest in Gryffindor values. This Slytherin might see it as a betrayal to their house and try to stop Regulus by any means necessary, even if it means using dark magic.
The Ministry of Magic could also be the main antagonist in this case. If the Ministry is corrupted or under the influence of dark forces, they could manipulate Harry's friends. They might convince his friends that Harry is a danger to the wizarding world order, and his friends, being naive or misinformed, end up betraying him.