Almost anything on the Shmoop module list would probably fit under the category of "literary fiction": it's an umbrella term for a story or novel that focuses more on character development and style than on page-turning plots. And it's this kind of fiction that you usually read for school: books that provoke discussion over what it all means (Life, the Universe, and Everything).
The Great Gatsby is definitely no exception. Fitzgerald is much more interested in plumbing the depths of Gatsby's heart and in experimenting with symbolic language (the green light, anyone?) than he is in working through the latest forensic evidence to give us clues for who hit Myrtle with his (or her) car. This novel is definitely not CSI: West Egg.
And the way Nick's narration jumps around, shifting from dialogue to personal meditation to foreshadowing and back again, tips us off that The Great Gatsby is also a Modernist work (like a lot of other books to come in the wake of World War I – check out any of our Shmoop guides on Ernest Hemingway or James Joyce novels for examples). It's fragmented and non-linear, but the writing style also tries to get at difficult truths that a more realistic book might not capture.