The Norse gods could be involved too. Odin, with his all - seeing nature, would be a likely candidate to be watching. He might be looking for signs of great warriors or leaders among the 'Descendants' characters. Thor could also be there, interested in the physical battles and the strength of the characters. Loki might be the most mischievous observer, perhaps secretly causing some of the chaos in the 'Descendants' world just for his own amusement.
Maybe the gods are some of the tailed beasts in a more divine form. Since they are powerful entities, they could raise Naruto, teaching him how to better control the power of the Nine - Tails within him and also how to communicate with other tailed beasts on a more spiritual level.
Poseidon would probably be one of the main gods watching. Since Percy is his son, he would have a strong interest in his well - being during the fight.
I think Poseidon is a must - he's Percy's father, after all. Then there's Zeus, who likes to be in control and would want to see this baby who could grow to be a significant force. Apollo might come just for the fun of it, being the god of many things including prophecy, he might have some insights. And Artemis, as the goddess of the hunt, could also be there to see what all the fuss is about regarding baby Percy.
In the 'akatsuki watch battle of gods fanfiction', the Akatsuki members are bound to be the main focus. This includes Sasori, Deidara, and Hidan. However, when it comes to the 'Battle of Gods' part, characters like Whis, who is Beerus' attendant, could also play a significant role as the Akatsuki observe the interactions and battles among these Dragon Ball characters.
Ra would be a likely candidate. He's one of the most important Egyptian gods, and his reaction to the fanfiction would be interesting. He might be offended if his power is misrepresented or amused if the writer has come up with a unique take on his role.
Athena might also be a good candidate. Although she and Poseidon have their differences, she could see Percy's potential and befriend him. She could offer him wisdom and strategic advice for his various adventures.
Aphrodite could also be one of the spying gods. Since she loves meddling in love affairs, and Percy and Annabeth's relationship is quite famous among the demigods. She might be spying to see how their relationship develops and perhaps add a little nudge here and there if she feels like it.
Obviously, Percy would be one of the main characters. And of course, the gods would play important roles too. Gods like Zeus, Poseidon, and Athena are likely to be there. Percy's friends from the demigod world, like Annabeth, might also make appearances.
Obviously, Percy Jackson himself. He's the one doing the pranking. Then, there would probably be the gods like Zeus, Poseidon, and Athena who are the targets of his pranks. His friends like Annabeth might also be involved, either helping Percy with the pranks or trying to stop him when they think he's going too far.
Percy is the central figure for sure. The gods, like Hades, might be there too if the story involves issues related to the Underworld. Grover could also play a role. He is Percy's friend and might be there to support Percy when he yells at the gods, or his experiences with the gods could be part of what drives Percy to yell. Chiron, the centaur, could also be a character as he is a mentor to Percy and could have some influence on the situation.