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.
The Sword God Realm is a Japanese light novel and animation work. The frequency of updates depends on the author and publisher. As for the author, the animation production company would release a new animation and a new chapter every quarter, while the novel author, Kawahara Rei, would release a new chapter on a specific day every month. However, due to the author's daily work and other factors, the update speed may be delayed.
Other than that, some other publishing companies would also release manga and light novel adaptation works from the Sword and Sword God Realm. These works might be updated at different times. Therefore, if one wanted to keep up with the latest content in the Sword God Domain, one could follow the author's and the publishing company's official social media accounts or official websites, or wait for the relevant media channels to report on it.
Well, in some fanfictions, Naruto might get the Buster Sword through a special encounter. Maybe he stumbles upon it during a mission in an ancient ruin. Another possibility is that it could be given to him by a powerful ally who has connections to the world where the Buster Sword originally comes from.
He could face powerful sword - using pirates. Since Luffy now has a sword, he would attract the attention of other sword - masters in the pirate world. These opponents would test his new - found sword skills and force him to improve quickly.
Sorry, I don't know when that happens. It could be tied to specific production timelines or licensing agreements. You might want to keep an eye on official announcements or related news sources.
It varies for different people. Some might think it gets good early on, while others might say it takes a while. It depends on your preferences for the story elements and gameplay.