In the long run, they might realize that they need to change their behavior. They could start thinking about how to make it up to Chichi. Maybe they'll go and do some nice things for her, like getting her some flowers or cleaning the house really well without being asked. This would show that they've learned their lesson and are trying to be better sons.
He might be a bit flustered at first. Gohan is not the most confident when it comes to romance. He could stutter a bit and then take a moment to compose himself before responding positively. He might say something like 'I've always hoped for this, but I'm just so surprised right now' and then go on to express his love for her.
Goku would be a mix of angry and worried. His Saiyan blood would make him want to rush into battle with Raditz right away. But he also knows how dangerous Raditz is, so he might first try to gather some allies, like Piccolo. He'd be constantly thinking about Gohan and how to rescue him as quickly as possible.
He might be surprised at first. After all, having his tail back is a big change. He could start to explore the new sensations and abilities that come with it, like better balance perhaps.
Well, there are also those characters who might be a bit shocked or confused. Take Levi from Attack on Titan. He's a very serious and no - nonsense type of character. If he were to read some of the fanfic about him that deviated too much from his own self - perception, like being made into a comical or overly emotional character in the fanfic, he might be taken aback at first. But then he might also realize it's just a different interpretation and not take it too seriously.
Goku would be extremely happy. He would see Gohan starting his own family and be proud of how far his son has come. He might also be excited about the potential for a new generation of strong fighters in the family. Chi - Chi would be overjoyed as well, seeing her son find love and start a new chapter in his life.
He could be very sad and withdrawn. After all, he expected love and support from his mother, not abuse. So he might start to isolate himself from her in the fanfic.
Some Avengers might be really confused at first. For example, Tony Stark would probably be intrigued yet find it a bit odd. He'd be analyzing it like he does with new tech, trying to understand the appeal and the different universes within the anime fanfiction.
There could be characters who are outraged. If the fanfiction distorts their personality or makes them do things completely against their nature, they may be very angry. For example, a noble character made to act like a coward in the fanfiction would likely not take it well.