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.
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, in Dragon Ball lore, Gohan's tail was a Saiyan trait. In this fanfiction, it might mean he has more access to his Saiyan instincts. Maybe he can transform more easily into a Great Ape or use some of the primal Saiyan powers that were associated with the tail.
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.
Perhaps it was the result of a special training or a new form of energy absorption. Gohan might have been exposed to a new type of ki or energy that stimulated the regrowth of his tail. For example, if he was in a place with a high concentration of a certain type of energy that interacted with his Saiyan cells in a way that caused his tail to grow back.
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.
In some fanfictions, Gohan might have a love interest within the Fairy Tail world. It could be an unexpected pairing, like with Lucy. Their relationship would be a mix of Gohan's more serious and reserved nature with Lucy's outgoing and adventurous spirit. This would create a lot of interesting dynamics, both in terms of their personal relationship and how they interact with the rest of the guild.
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.
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.