Ash's other Pokemon like Bulbasaur, Charmander (or Charizard if it has evolved), and Squirtle would also be very concerned. They have been through a lot with Ash and would not understand his sudden change. They might try to communicate with him in their own ways. Bulbasaur could use its vines to hold Ash back from doing something bad under Team Galactic's influence. They would all be united in their goal to bring the old Ash back.
His newer Pokemon like Greninja would be extremely conflicted. Greninja has a deep spiritual connection with Ash. It would sense that something is not right but also might be torn between following Ash's lead and its own instincts. It could try to reach out to Ash on a deeper level, perhaps through their shared ninja-like bond, to remind him of who he really is and what he stands for.
Pikachu would likely be very confused and resistant. Pikachu has a strong bond with Ash and knows his true nature. It would probably try to shock Ash out of whatever trance or false belief he has from joining Team Galactic. For example, when Ash starts to follow Team Galactic's orders that go against his usual principles, Pikachu might refuse to cooperate and try to show Ash that he's on the wrong path.
Pikachu would probably be very confused at first. It's used to fighting against Team Rocket, not being on their side. It might resist and try to zap Ash to snap him out of it.
One possibility is that Ash joins Team Galactic under some sort of misunderstanding. Maybe he thinks they are a group with good intentions at first. Once he realizes their true nature, he has to figure out a way to escape or stop them. This could create a lot of tension and action - packed scenes in the fanfiction.
Some of his more loyal Pokemon might try to resist the change. For example, Pikachu could use electric shocks to try and jolt Ash back to his senses. Pikachu has a strong bond with Ash and would be very confused and sad about his decision to join Team Rocket.
One possibility could be that Ash is initially deceived by Team Galactic's false promises of creating a better world for Pokemon. He might be drawn in by their grand talk about power and a new order. For example, they could convince him that their plans for using the power of the legendary Pokemon would benefit all Pokemon in the long run. However, as he spends more time with them, he starts to notice their more sinister motives, like their disregard for the balance of nature and the harm they are actually causing to Pokemon habitats.
Ash joining Team Galactic in fanfiction could mean a complete shift in his character arc. He might be forced to do things against his will at first. But as time goes on, he could use his knowledge of Pokemon to gain an upper hand within the team. For example, he could use his bond with his Pokemon to resist the team's orders that are harmful to Pokemon.
If May joins Team Galactic in the fanfic, her character would go through significant changes. At the start, she might be curious and excited about being part of something new. But as she realizes the true nature of Team Galactic, she would experience a lot of guilt for being associated with them. This guilt would drive her to try and make amends. She would become more determined and勇敢 (brave in Chinese, for lack of a better English equivalent in this context). Her relationship with her Pokemon would also change, becoming stronger as they support her in her fight against Team Galactic.
Well, if May joins Team Galactic in the fanfic, she might be used as a pawn at first. Team Galactic could see her as a way to get to other trainers or Pokemon. However, May is a strong - willed character. So she could start to secretly undermine Team Galactic from within. For example, she might feed false information to them or help other Pokemon escape their clutches. Eventually, she would probably lead a rebellion within the team to expose their wrongdoings.
His personality might become more positive. Instead of being a bit of a schemer like he was with Team Rocket, he could become more helpful and honest. He might start to care more about others' feelings.
His Pikachu would probably be very supportive. It would stay by his side as he studies, maybe even help him in small ways like fetching medical tools. It trusts Ash completely and would be happy to see him pursue something new.
Well, if Ash joins Team Rocket in a fanfic, it would be a huge shift in the Pokemon world. His friends, like Misty and Brock, would be shocked. They might try to rescue him. Ash, on the other hand, could initially be attracted to the idea of a new adventure with Team Rocket. He could see them as underdogs in a way, always failing but still trying. But as time goes on, he'd start to see the problems with their way of doing things.