I'm not entirely sure if there is a specific fanfiction with exactly that premise. However, given the popularity of both Naruto and the concept of sealing powerful entities in Naruto's world, it's quite possible. Fanfiction writers love to experiment with combining different fictional elements, so it wouldn't be surprising if someone has written a story where Jenova is sealed in Naruto. You can try searching on popular fanfiction websites like Fanfiction.net or Archive of Our Own to find out for sure.
In many such fanfictions, it could be a variety of deities. It might be an ancient and powerful god from Naruto's world's mythology that the story's author has chosen to insert. For example, it could be a god of elements like a super - powerful fire god, which could give Naruto even more control over his existing fire - based jutsu and new, more extreme abilities related to fire.
Well, one reason could be that the author wants to explore an alternate universe. Maybe in this fanfiction's version of the Naruto world, the events leading to the sealing of the Kyuubi never occurred. It could be due to different political situations among the ninja villages, or perhaps a different chain of events that prevented the Kyuubi's attack on Konoha in the first place. So, without the attack, there was no need for the seal.
With his enemies, it would be a whole different story. They might be more cautious about attacking Naruto. Some might even try to find a way to use the god sealed in him for their own purposes. And Naruto's relationship with the hidden villages could also change. They might see him as a potential threat or a very valuable asset.
Well, Naruto might become more mysterious. Since Gaster is an unknown and rather enigmatic entity, Naruto could start having strange thoughts or powers that he doesn't fully understand at first. He might seem more withdrawn as he tries to figure out what's going on inside him, and his normal cheerful and straightforward personality could be clouded by the presence of Gaster.
His abilities would change in multiple ways. Firstly, his chakra control might be affected. Since Godzilla is a massive and powerful entity, it could disrupt Naruto's normal chakra flow. However, he could also learn to harness Godzilla's power within him. For example, he might be able to use Godzilla's raw strength to enhance his Sage Mode or even his Nine - Tails transformation. This could make him an even more formidable opponent. His speed could also be enhanced as Godzilla's power might give him a boost in his physical capabilities, allowing him to move faster and react quicker in battle.
One idea could be exploring Naruto's growth not just in terms of power from the Kyuubi, but also his emotional and mental growth. For example, how he overcomes the stigma of being the Kyuubi's host among the villagers who once feared and hated him. This could involve his relationships with characters like Iruka, who was one of the first to see Naruto as more than just a container for the Kyuubi.
One possible consequence is that the power balance in the ninja world is completely disrupted. With the Kyuubi's massive chakra and power running free, smaller villages might be constantly in fear of being attacked. Konoha, in particular, would have to find new ways to defend itself without relying on the fact that the Kyuubi is sealed within one of its shinobi.
He could gain the power of manipulation. Gaster might give Naruto the ability to manipulate space or time on a small scale. Maybe he could create small portals or slow down time for a short period during combat.
In such a fanfiction, Naruto might gain some extraordinary powers related to the phoenix. For example, he could have the ability to heal quickly like the phoenix's regenerative powers. He may also be able to use fire - based attacks that are far more powerful and unique compared to his regular jutsu.