Vegeta would probably be both jealous and intrigued. Jealous because Goku has achieved a new level of power through this fusion, and intrigued because he wants to see how this new being fares against enemies. He might also be secretly glad that Frieza, his long - time enemy, is now in a form that he can tolerate (barely).
His father might be extremely proud. After all, Goku has achieved so much, like protecting the Earth multiple times. He would look at Goku with admiration for his strength and kindness.
Well, Goku's super strength and fighting abilities could make him a powerful ally or a formidable foe in the Naruto world. His ki powers might be seen as a strange new form of chakra. He could also teach the Naruto characters some of his unique fighting techniques, like the Kamehameha wave.
Well, Goku's extreme power level could make him a sort of legend in the Naruto fanfiction world. His Saiyan abilities like the Kamehameha and Super Saiyan forms would be completely foreign to the ninjas. He might be seen as an otherworldly being, and his presence could disrupt the power balance. Maybe he'd come to the Naruto world seeking a new challenge, or perhaps he was accidentally transported there.
If Goku were in the DC Universe in a fanfiction, it would be really interesting. He might form an alliance with Superman. They both have a strong sense of justice. But their fighting styles are quite different. Goku's martial arts skills combined with Superman's super - powers could make for an unbeatable team against the likes of Darkseid or other major threats.
One idea could be that Goku arrives in the Pokemon world by some sort of interdimensional rift. He might be confused at first but then start to explore the concept of Pokemon battles. He could use his martial arts skills to train Pokemon in a unique way, like teaching them special combat moves. For example, he could train a Pikachu to use a move similar to his Kamehameha, but in a more Pokemon - friendly version, like an electric blast with a similar charging motion.
Gohan would be really surprised, but being the kind - hearted person he is, he would try to understand and help out. He could offer to train with them in a more gentle way to keep them fit during the pregnancy. Piccolo would be more of a silent observer, but still be there in case of any threats.
The reaction among fans would be quite mixed. Some die - hard fans of the original series would be appalled as they have a clear image of ChiChi and Goku's relationship. They would see this type of fanfiction as a perversion of the original concept. On the other hand, there are those who are more open - minded and might see it as an interesting, if not controversial, exploration of the characters. But overall, the negative reactions would probably outweigh the positive ones.
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.