Yes, it is. As a 'Code Geass' fan, it gives you more content related to the series. You get to explore the story further and might discover new aspects of the characters and the world.
Code Geass did start as a manga. However, its anime version became even more widely known and beloved. The manga provided the foundation for the rich story and characters that were further developed in the anime.
Code Geass started as an anime. The manga was based on the anime's success and came after to expand the story for fans. Some elements might differ between the two, but the anime was the original form.
The 'Code Geass 5 Year Fanfic' might expand on the original world - building by delving deeper into the politics of the different nations. For example, it could explore how Britannia's rule has changed over the five years, whether there are new rebellions brewing in different regions. It may also focus on the economic aspects. Maybe some regions are suffering more due to Britannia's policies, and this leads to new forms of resistance.
The Code Geass series includes many works, and the most popular ones include:
1 Code Geass: The Rebellion of Lelouch (1999): A long-form animation composed of multiple media forms such as animation, manga, and novels that tells the story of the struggle between the protagonist Lelouch and the rebels.
2 Code Geassass Zero Witch (2006): This is an anime adapted from a novel that tells the story of the struggle between the female protagonist Snow Rabbit and the Witch Sect.
3 Code Geassass Dragon's Eye (2009): This is an animation adapted from a novel that tells the story of the struggle between the protagonist Lelouch and the Red Stone Party.
In addition, Code Geass also had a number of novels and comics, some of which were adapted into anime.