Well, yes, but is it truly possible to be outside any genre? Particularly with the modern tendency to open up new genres and subgenres at the drop of a hot, how many books can truly fall outside of all of them?
Presumably, general fiction contains no fantasy, science fiction, or supernatural horror. So we could see realistic fiction. But if it's too literary, it's lit fic. Too much action, and it's action/adventure or thriller. Any attempt to solve a crime, and it's mystery, police procedural, or something like that. If there's a military angle, it's military fiction. If it's too frightening, it's psychological horror. If it's funny, it's comedy. If there is much sex or love, it's some kind of romance. If it takes place in the past, it's historical fiction. You can see where I'm going with this. Most things that are even remotely exciting have their own genre.
I searched for general fiction on Amazon, and all the results I checked were something else. (None of the visible Amazon categories were general fiction. Mostly, they were historical or romance, but books of practically every genre turned up.)
In other words, as far as I can tell, it's a BS category publishers use when they know full well where a book fits but want to put it somewhere else--like a book they know is romance, but they don't want it associated with the romance stereotypes.
https://book-genres.com/general-fiction-definition/ defines general fiction essentially as hybrid fiction--books that straddle at least two genres, and probably three or more. But their own list contains titles that are clearly something else. And frankly, a broad category that covers every book that's a hybrid is not useful for readers. What good is a category that, at least according to genre.com, includes the Harry Potter books, the Narnia books, the Lord of the Rings (pretty much all fantasy, with the first two being for younger readers) East of Eden (Amazon says family saga and classic fiction), the Great Gatsby (Amazon says classical fiction and literary fiction), and Lolita (Amazon says classic American fiction and psychological fiction). Put another way, how much is Harry Potter like Lolita?
Genres and subgenres should give you some idea of what to expect. General fiction is too broad and ill-defined to do that.