The gods might be outraged. They could see Percy as one of their own, especially since he has done so much for them. They might immediately seek to punish the abuser. For example, Zeus could send a bolt of lightning towards the person who abused Percy.
Well, some past gods might be amused. They'd see these modern interpretations of their stories and characters in the Percy Jackson fanfiction as a strange yet interesting new form of worship or at least attention. For example, Zeus might be a bit irked at some of the liberties taken with his character, but also secretly flattered that people are still so interested in him.
Initially, Naruto would be heartbroken. His friends and his village mean everything to him, and the thought that the gods, who are supposed to be just, have turned against him would be a huge blow. However, he wouldn't give up. He would use his determination and his unwavering belief in himself. He might start looking for allies among those who are also disillusioned with the gods or who believe in his innocence.
Well, in many such fanfics, the gods might react with anger. They could seek revenge on the abuser. For example, Zeus might send a bolt of lightning to strike the place where the abuse took place as a sign of his wrath.
Most of the gods would be confused at first. Hera might see it as an opportunity to manipulate the situation for her own ends. She could try to use Percy's new connection to Ares to create more chaos or to further her own plans. Poseidon would be very conflicted. On one hand, he has a connection to Percy as his son, but now Percy has this new link to Ares which could put Poseidon in a difficult position among the other gods.
At first, Percy could be in shock. He can't believe that the gods, whom he has served and trusted, would betray him. But then, as the reality sinks in, he might become determined to get justice. He starts to plan his next moves carefully, using his wits and the skills he has learned over the years. He may also feel a sense of sadness and loneliness, as he realizes he is now on his own against the powerful gods.