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.
One Piece was a very large comic and animation work, which described Luffy's family story in a very wide range of plots. Luffy's father was Admiral Zoro, his mother was the pirate Nami, and he had two older brothers, Choba and Franky, as well as an older sister, Nicole Robin. In One Piece's storyline, Luffy and his family experienced many adventures and challenges in pursuit of the dream of becoming One Piece.
One Piece's middle lane character Fei Xueba was furious. In the sixth episode," Luffy VS One of the Four Emperors, King of Beasts," Luffy learned to be domineering in his battle with White Beard, one of the Four Emperors.