Gabi is still alive in the AOT manga. Her fate was determined by the plot and the author's choices, and death wasn't one of them for her in this particular storyline.
It's fairly common. Fans often look for more content related to their favorite shows. Since 'Attack on Titan' has a large and passionate fan base, there are a lot of fanfics available. 'Aot' fans might watch (read) these fanfics to explore different scenarios that the official story didn't cover. For example, they might want to see how a certain character would react in a different situation or what would happen if a major plot event had a different outcome.
Gabi remains alive in the manga. The plot doesn't lead to her death. It's one of those character arcs where she continues on despite facing various challenges.
There could be various fanfictions. Maybe some that focus on alternative endings, like ones where certain characters survive who didn't in the original. For example, a fanfiction where Erwin survives and leads the Scouts in a different way.
For a more villainous touch, including Voldemort or his followers could create high - stakes situations. Bellatrix Lestrange could be a terrifying adversary for Harry and Gabi. On the other hand, adding new original characters, like a powerful but reclusive wizard who lives in the Forbidden Forest, could also make the story more exciting.
One great Niccolo x Sasha fanfic could be 'A Culinary Bond'. In this fic, Niccolo's cooking skills are used to bring him closer to Sasha. Their relationship develops as he creates special dishes just for her. Another might be 'Under the Stars', where they have a chance encounter while stargazing and start to realize their feelings for each other.