Luffy uses Gear 5 in a significant battle towards the later part of the manga. But the exact chapter or arc can vary depending on how you're reading it.
I think it's in a relatively recent chapter, but the exact one might vary depending on where you're reading the manga. You might need to check the latest updates.
Well, in the fanfiction, Gear 4 Luffy might have had some new and unexpected interactions. Maybe he could have teamed up with some other pirates in a different way. Since he was in such a powerful form, he could have inspired more of the pirates to fight back more bravely against the Marines. And his presence in Gear 4 could have also affected the admirals' strategies, making them more cautious or forcing them to use their more powerful techniques earlier.
It would likely make the storyline more exciting. Gear 4 Luffy is extremely powerful, so his presence in Marineford would change the power dynamics. He could defeat more enemies quickly.
There were a few novels that fit this description, such as Legend of the Earth Emperor, Mountain God in the City, Incense Becomes God, I Actually Became a Mountain God, and so on. You can choose one according to the plot introduction of these novels and your own taste. I wish you a happy reading!
The theme of acceptance is quite common. Fem Luffy often has to be accepted by the other characters in the story. This could be because she's different due to her gender or her slightly different personality traits. Also, the theme of self - discovery is prevalent. As fem Luffy navigates the pirate world, she discovers more about herself, her abilities, and her place in the grand scheme of things.