I had this book as my prescribed text for History & Memory, and to be honest, I didn't fully get the title of the book and it's reference in the book. I believe that each gate is meant to symbolise a challenge along the way that must be passed and of course the 50th Gate is the last that must be passed. A good quote to help understand it a little bit is the opening and closing line of the novel.
I didn't mention the title once, or refer to it, apart from just naming the text and got 18/20 as my raw mark for this essay in the HSC English Advanced Paper, so don't stress if you don't get the contextual reference.