We won't argue with you if you want to call this story an example of magical realism. (Magical realism is a genre in which magical or supernatural elements appear in the text alongside perfectly ordinary ones.) After all, it has inspired a gazillion sci-fi and fantasy authors to write stories about parallel universes, an idea that does seem fairly magical.
We'd like to point out, however, that the term "magical realism" doesn't even begin to capture the many sides of "The Garden of Forking Paths." It's also a spy story, a mystery, and a psychological thriller. It's a treatise on philosophy and arguably an early example of postmodern fiction. Above all else, it's a classic, which is why we've also categorized it under "literary fiction," a sort of catch-all term for literature that's just too great to fit anywhere else.