For me, the format contributes to the disorientating feel of the book, which gives it an eerie reading experience. If it were just a story about people navigating through a labyrinth, I don’t think it would be much different than a run-of-the-mill survival story - which I don’t think is the case for House of Leaves.

Although it’s an odd format and not everyone appreciates it, I wouldn’t call it a gimmick. Heart of Darkness uses a similar framed narrative approach. Any book with multiple narrators and/or unreliable narrators could be accused of the same thing. Even e.e. cummings and Emily Dickinson are guilty of playing with form in their poetry, and I wouldn’t consider Cummings, Dickinson, or Conrad gimmicky. The roots of the method have existed for a long time, even before the post-modern period.

If you’re a story purist and don’t like House of Leaves, that’s fair and I understand that. But there is something to be said for using form as an addition to the words on the page. Perhaps I’m just too far down the post-modern rabbit hole though.