The creation of One Piece manga involves a lot of steps. The mangaer first conceives the overall plot and characters, then does detailed storyboarding and drawing. They also have to consider the feedback from readers and the market to make continuous improvements.
There's no way to tell for certain who made the 'gavin oc one piece fanfic' without more information. It could be an amateur writer who just wanted to explore their own ideas in the One Piece universe. Maybe it was created by a small group of friends who were discussing what an original character like Gavin could do in the One Piece world and then decided to write it down as a fanfic.
One Thousand and One Nights was an epic fantasy novel written by Sāfī Hīdayl-Allāh in 1399. It spanned centuries and told many myths, legends, and adventures. This novel is considered one of the masterpieces of Arabic literature and one of the most popular fantasy novels in the world.