Well, he'd probably be super OP. He could transform into all kinds of alien forms and use their powers in his battles. Imagine Naruto using Heatblast's powers to fight against Akatsuki members. It would be a whole new level of combat in the Naruto universe.
In this fanfiction, Gwen might use the Omnitrix to fight new and unexpected villains. She could transform into various alien forms and discover new powers within each transformation.
One possibility is that the story could explore how Gwen's personality changes when she has the Omnitrix. She might become more confident or more responsible. Also, there could be new villains who are attracted to the Omnitrix now that Gwen has it. She would then have to face these threats and protect the Omnitrix from falling into the wrong hands.
The story could start with Gwen unexpectedly getting the Omnitrix. Then, she might start exploring the different alien forms she can transform into. She'd likely test out their powers in various situations, gradually getting a handle on how to use them effectively.
One interesting concept could be how the Omnitrix interacts with the unique chakra system in Naruto. Maybe it could transform the user into Naruto characters with added alien abilities from the Omnitrix. For example, turning into Naruto with the powers of Heatblast, allowing for a super - charged Rasengan with fire - based attacks.
In this fanfiction, Naruto might use the Omnitrix to gain various alien forms' powers. He could use those powers to fight stronger enemies in the Naruto universe, like the Akatsuki members. Maybe he would use the powers for stealth or super strength depending on the alien form.
Naruto himself is often very popular. Since it's related to his world, fans like to see how he would interact with the Omnitrix and its powers. He could use it in his journey to become Hokage, or to protect his friends in new and exciting ways.