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.
The Straw Hat crew would probably be very surprised at first. Nami might worry about how Hancock's presence would affect their finances, since Hancock is used to a more luxurious lifestyle. Usopp would be in awe of Hancock's beauty and powers. Chopper would be excited to have Hancock around, as she could potentially teach him about different kinds of medicine from Amazon Lily. Zoro might be a bit skeptical at first but would come to accept it as long as Luffy was happy.
The Straw Hat crew would be in for a big shock at first. But then they would quickly rally around Luffy and Nami. Robin might offer her wisdom on raising a child and share stories from her own past. Franky would build amazing things for the baby like a super - safe crib or a high - tech stroller. Brook would play music to soothe the baby. And they would all work together to make sure the baby has a great life on the ship.
Usopp might be inspired to be braver. When he watches Luffy's memories, he could see how Luffy fearlessly faces powerful enemies. This would make Usopp want to overcome his own cowardice and be more like Luffy. And Robin, she would gain more insights into Luffy's kind heart. She might see how Luffy has always been inclusive and accepting of everyone, which would make her feel even more at home in the crew.