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.
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.
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.
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.
One possible plot could be that Gear 5 Luffy arrives at Marineford during the big battle. He might use his newfound powers to turn the tides of the fight. For example, he could use his rubbery abilities in even more extreme ways, like stretching to deflect cannonballs aimed at his allies and then bouncing them back at the enemy ships. His presence would shock both the Marines and the pirates. He could also engage in a one - on - one fight with some of the top Marine officers in a more comical yet powerful way, as Gear 5 gives him a more carefree and powerful fighting style.
For a Luffy Gear 5 fanfic, you could have a plot where Luffy's Gear 5 form is somehow corrupted by a dark force. He starts to lose control of his actions and becomes a danger to his friends. His crew has to find a way to purify his Gear 5 power. Another interesting idea is that Luffy discovers that Gear 5 has a hidden ability that allows him to travel through time briefly. He uses this ability to go back and change a crucial event in the past that has far - reaching consequences for the present. This could lead to a whole new set of adventures and challenges for Luffy and his crew.
In some One Piece Luffy Gear 5 fanfiction, there could be the idea of Luffy sharing his Gear 5 power with his crewmates temporarily. This would not only make for some cool team - up attacks but also show the deep bond within the Straw Hat crew. Also, a fanfiction might explore how Luffy's Gear 5 affects his relationship with other pirates or the World Government. Maybe it makes him an even bigger target or maybe it causes some pirates to want to ally with him out of fear or respect.