To start writing gamer fiction adaptation fanfiction, you should play the game (if it's based on a game) or reread the original fiction multiple times. This helps you get a feel for the tone, the language, and the overall vibe. Next, think about what you would like to change or add. It could be a new love interest for a character, a new type of magic in the game world, or a different ending to the story. Once you have your idea, create an outline. Outline the main plot points, the new characters (if any), and how they will interact with the existing elements. Then just start writing.
Begin by identifying your motivation. Are you writing because you want to explore a different side of a character, or because you think the original story could have been better in some areas? Once you know your motivation, do your research. Look at other fanfictions in the same genre to see what works and what doesn't. Then, start with a small idea. Maybe it's just a short scene that you can build on. For instance, if the original gamer fiction has a battle scene, you could start by writing a different version of that battle from the perspective of a minor character. As you write, let your creativity flow, but always keep in mind the rules and essence of the original gamer fiction.