Unfortunately, it would be considered under the massive fan fiction umbrella simply because it’s not 100% your story and world. Fan fiction does get a lot of negative connotations, but that’s mostly because of the celebrity fan fiction you often see and hear about, or the badly written romances—things like After and Fifty Shades of Grey, both of which were originally fan fiction: After being about Harry Styles, and Fifty Shades being about Twilight (Bella and Edward, I suppose?)
But there’s a lot of different types of fan fiction out there, and there’s also a massive fan base for it too. I constantly scroll through BookTok and hear about fan fictions everywhere, whether they’re from Wattpad, A03, or elsewhere. Some major authors even got into writing with fan fiction and still read it, even at 30 years old. Is there a bad reputation with them? Sure. But there’s a bad reputation with every genre at some point. I mean, look at romance and erotica. People constantly look down on them, judge you for writing or reading it, and more. And yet, those two are the number one bestselling genres of all time.
The thing is, though, is that writing a novelization on a video game (specifically one you haven’t gotten the permission to write for and had the thumbs up from the developers to get ideas on what to put in it, etc. to make it all canon) is fan fiction. It’d be completely different if they gave you permission and even got it published, because then, it would be part of the whole storyline, etc. Think of it like the Avatar: the Last Airbender comics and novels. They’re not fan fiction because they’re continuing the stories of the characters, they got the permission, and everything is canon, especially since they added bits and pieces from the comics into the universe of Legend of Korra that even set up the setting. But if they were in your shoes where they weren’t getting published and had the OGs names on it, it would be considered fan fiction, and seeing what happens to the gang wouldn’t be considered canon at all.
But if you really don’t want your story to be associated with fan fiction, you can make it original by not making it a full novelization of the video game. You can write a story that is inspired by the video game. For example, I could write a story that’s inspired by the Forest (which a horror survival game about a man being stranded on an island after being in a plane crash, trying to find his lost son, and trying to survive a village of mutated cannibals) where someone is on an island, trying to escape it and get help, but also finding the dark truths and secrets that hide within the islands caverns—a village of mutated people after they all got infected with some unknown disease.
Actually, this sounds like a great book lol. You could, technically (or possibly) get away with such a plotline, especially if you try to twist the video game into your favor so yours sounds different. You’re still nudging to the video game, it’s not a fan fiction, and it’s 100% yours. Wins all around. But if you do decide to go this route, you do want to make it well known that the story was inspired by said video game just so people who know of the game aren’t thinking you’re ripping it off.