I think Erza would be a close ally. She is a very strong and disciplined fighter in Fairy Tail, much like Gohan. She would respect Gohan's power and his sense of honor. They could fight side by side, with Erza using her various armors and swords, and Gohan using his Saiyan abilities. They would both be able to strategize well in battles and protect the other guild members. Also, Gohan could learn a thing or two about perseverance from Erza, and she could be inspired by his otherworldly powers.
Gray could also be a great ally for Gohan. Gray is a cool - headed fighter, and his ice - make magic could complement Gohan's attacks. For instance, Gohan could use his ki to heat up the area while Gray freezes the enemies. They could also bond over their training regimens. Gray's constant training to improve his ice - make magic is similar to Gohan's Saiyan training. They would both be able to push each other to become even stronger and would make a formidable team when facing the threats to the Fairy Tail guild.
Probably Natsu. They both have a strong sense of justice and are very powerful fighters. Natsu's fire - based attacks and Gohan's ki blasts could make for some amazing combined attacks in battle.
Captain America could be a great ally. His sense of justice and leadership would mesh well with Gohan's own values. They could both lead a team into battle and inspire others with their courage.
The plot could involve Gohan getting transported to the Fairy Tail world. Maybe he'd join the guild and use his Saiyan powers in their quests. He could team up with Natsu and the gang, and his super strength and ki blasts would be a huge asset in battles against the dark guilds.
I think Captain America would be a likely ally. Captain America is all about justice and doing what's right, which aligns well with Gohan's values. They both have a strong moral compass and would respect each other's fighting abilities.
Gohan's powers could be a great addition to the Fairy Tail universe. His Saiyan strength and ability to go Super Saiyan would make him one of the most powerful mages in the guild. He could use his ki blasts like powerful magic attacks, and his speed and agility would be useful in battles against dark guilds.
Gohan's powers would be quite unique in the Marvel Universe. His Super Saiyan forms would give him a massive boost in power, far beyond what most normal Marvel characters can achieve without some sort of external boost. His Zenkai boosts, which increase his power after near - death experiences, could also be an interesting concept to introduce. And his martial arts skills, combined with his Saiyan instincts, would make him a very adaptable fighter against the various threats in the Marvel world. He could also potentially teach other heroes his fighting techniques.
One possibility is that it could be a genetic anomaly. Maybe something in his Saiyan genes that was dormant suddenly activated, causing the tail to grow back. In the Dragon Ball universe, Saiyan genetics are complex, and there could be some unexplored factors at play.
It could be a new villain created specifically for the fanfiction. Maybe a powerful sorcerer who wants to steal the Saiyan's power.
Naruto would probably be the most interested. He loves making new friends and Gohan's power would surely catch his attention. He'd be eager to learn about Gohan's world and his abilities.
One interesting aspect is the power combination. Gohan has his Saiyan powers like super strength, speed, and ki blasts, while Fairy Tail characters have their unique magics. When these two different power systems interact in fanfiction, it creates really cool battle and adventure scenarios. For example, seeing Gohan use his ki to enhance a Fairy Tail mage's spell or vice versa.
I'm not sure of a specific one off the top of my head, but you can try searching on fanfiction.net. There are usually a lot of great stories there. Just type in 'Gohan Fairy Tail' in the search bar and you'll likely find some interesting reads.