Goku would react with pure determination. He'd use all of his skills and powers to track Raditz. His love for Gohan is so strong that he'd even consider pushing his own limits, like going Super Saiyan if he could at that point. He'd be focused solely on getting Gohan back from Raditz's clutches.
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.
One plot idea could be that Gohan's friends, like Krillin and Bulma, team up to rescue him. They might use some of Bulma's inventions to track Raditz. Another idea is that Gohan manages to outwit Raditz during the kidnapping. Maybe he uses his hidden power to escape Raditz's clutches briefly and leaves a trail for his father Goku to follow.
They'd be shocked. Goku is a legend in the Dragon Ball world, so they'd be super excited and probably a bit scared too.
Piccolo might also be a savior. He has a soft spot for Gohan since they trained together. He could use his strategic mind and powerful attacks to outwit Raditz and rescue Gohan. Maybe he teams up with Goku for an even more epic rescue mission.
Goku and Grandpa Gohan's interaction could be full of wisdom - sharing. Grandpa Gohan was a wise man, and he would pass on more of his knowledge to Goku. Goku, in turn, would tell Grandpa Gohan about all the new friends he has made and the different planets he has visited. They might also spar a bit, just like in the old days, with Grandpa Gohan giving Goku some pointers.
If Raditz survived in fanfiction, he would likely have a complex relationship with Goku. He might start to see the value in Goku's way of life. Goku could try to reform him, and Raditz could slowly become a more positive force in the Dragon Ball Z universe.
Naruto would be extremely enraged. He would vow to rescue Sakura no matter what. He would immediately start gathering his friends and form a rescue team. Sasuke, on the other hand, might initially seem more reserved but would also be determined to get Sakura back. He would use his Sharingan skills to track Madara.
Lydia would probably be very scared at first. After all, being kidnapped by a strange and unpredictable character like Beetlejuice is no joke.
Inuyasha would also be worried about Kagome's safety. He knows how devious Naraku can be. He might call out to Kagome as he gets closer to Naraku's lair, hoping to hear her voice and know that she's still alive. His reaction would be a mix of anger, worry, and determination to rescue the woman he loves.
Alex would probably be both scared and defiant at first. He's a brave kid, so he'd likely try to find a way to escape right away. He might also try to talk his way out, using his intelligence to reason with Yassen.
Percy would likely be both angry and calculating. He'd be angry because he's been kidnapped, but he'd also start thinking about how he can use his powers and skills to get out of the situation. He might try to reason with Khione if he thinks it could buy him some time to plan an escape. He'd also be looking for any weaknesses in Khione's defenses or in her personality that he could exploit.