People like to gatekeep it because it's unique in its formatting and style.

Basically it's a story with a ton of nested framing devices. You have:

• A guy named Navidson whose house keeps changing its geometry, until one day an infinite, sprawling series of corridors opens up, which he then explores with a team of professionals. Navidson is a jealous asshole who constantly gets into ego battles with every man around him.
• A guy named Zampano, who is writing a pseudo-academic analysis of Navidson's journey, stating that it all happened in a movie. Zampano is a paper-thin character who is only really incidentally connected to the story and doesn't get a ton of development until you're far into the novel.
• A guy named Johnny, who finds Zampano's writings after he dies and reads them/adds footnotes. Johnny is our main perspective character and is, imo, an irredeemable womanizer who probably has schizophrenia. I swear half his scenes are him boning some random woman he just met, and the other half are word salad or incoherent stories that draw vague parallels to Zampano's writings.
• "The editors", who are compiling all of these notes and footnotes into a book with appendices, which is canonically what you're reading as the end user.

The story unfolds in a way that invites you to read through footnotes, appendices, and sprawling side-narratives to a point where you can just get lost in random threads. Some people really like that, I find it annoying. It would be more involving if the characters were better. Even the side characters are mostly just assholes or set dressing, and every woman in the story is a loose slut to a point where it's weird.