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.
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.
Luffy Gear 5 Fanfic: In the heart of a new island, Luffy stood tall in his Gear 5 form. His body pulsed with a vibrant red energy as he faced the powerful new foe. 'I won't let you harm my friends!' he roared. With a swift motion, he stretched his arm like a rubber band, then snapped it forward, sending a shockwave of Haki towards the enemy. The foe tried to dodge but was caught in the blast, sent flying backwards. Luffy then bounced around the battlefield, his movements a blur, leaving trails of light as he continued his attack.
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.
One interesting idea could be exploring new abilities Luffy discovers within Gear 5 that he hasn't shown in the official story yet. For example, he might be able to manipulate his surroundings in even more creative ways, like turning the air into rubbery substances to create platforms or traps.
In fanfiction, Luffy's character in Gear 5 might develop a greater sense of responsibility. Since Gear 5 is so powerful, he realizes he has to be more careful about using it. For example, he might avoid using it in areas where there are a lot of innocent bystanders.