Percy could be really angry. He might lash out at Annabeth and those involved. He has a fiery temper sometimes, and this would be a huge betrayal.
He could also be very sad and heartbroken. He's deeply in love with Annabeth, so this would be a crushing blow. He might isolate himself from everyone and just wallow in his pain for a while. He might also question all the memories they had together, wondering if they were all lies.
She might be disgusted. Gabe is such a horrible person in Percy's life. So Annabeth could show immediate anger towards Gabe just from Percy's description.
Well, one possible reason could be that Percy made a really stupid decision without consulting Annabeth first. Like, if he went off on a dangerous mission on his own when they were supposed to be a team. Annabeth is a very strategic person and she might feel betrayed or extremely frustrated by such actions.
It could also be due to some sort of magic or mind - control. A powerful enemy might have cast a spell on Annabeth, forcing her to turn against Percy. In this case, it's not really her own will but the result of external manipulation.
She could be extremely worried. In fanfiction, Annabeth might constantly fret over Percy's safety as Styx is a very dangerous place. She may try to find ways to reach him or send help.
She might be a bit shocked at first. But then, being Sally, she'd probably quickly regain her composure and give them a knowing smile. After all, she's seen Percy grow up and she would be happy for him to have such a close relationship with Annabeth.
Annabeth could be under some kind of magical influence. In the Percy Jackson universe, there are many magical elements. She could be cursed or charmed without her knowledge. This would be a very interesting take in a fanfiction as it would add an element of mystery and also give her a chance for redemption once the truth is discovered. Also, it could be that she has some internal insecurities that she hasn't dealt with and they manifest in this way, making her act out of character.
One possible reason could be a misunderstanding. Maybe she saw Percy doing something that she misinterpreted as a betrayal of their friendship or their cause. For example, if Percy was seen talking secretly with an enemy, Annabeth might jump to conclusions without knowing the full story.
Percy would probably be extremely hurt. He trusts Annabeth deeply, so this betrayal would shatter that trust. He might feel angry at first, wanting to confront both Annabeth and Luke.
One possible reason could be jealousy. Maybe Percy gets more attention or recognition in their fictional world that makes Annabeth envious and thus appear to hate him.
In the fanfiction world, their personalities might be very complementary. Annabeth is intelligent and strategic, while Percy is brave and impulsive. This yin - yang dynamic could lead to a strong attraction, and the want to kiss could be a natural outgrowth of that attraction.