Giovanni's Room is told by a man named David, from a house in the south of France. There are few novels better suited to the verb "unfolds" than Giovanni's Room; David's narration moves more or less linearly, but at times he will tell events out of order or break into discussions of his present state. The result is that we know what happens by the fifth paragraph of the story. The mystery is how it happened.
There are a couple of those magical fictional assumptions underlying the way that David narrates the story. For one, he speaks in the present tense despite the fact that he is alone and wandering about the house. To whom is he speaking? Where are the words coming from, if he's not sitting there writing them down? For another, despite the fact that he is remembering things, David will often narrate them in great detail, as if he remembered exactly how it happened. At other points, he will break into a series of perhaps statements, but it seems odd that he never requires these for the most dramatic points in the story, even if he was not there.
In his final fight with Hella, David tells her that he wasn't lying to her. As he says, "I was lying to myself" (2.5.107). As a reader, your ears should perk up with this line. You might even make a face and groan a little because what David is admitting here, to the reader as much as to Hella, is that he's not a reliable narrator. It's not that David would intentionally distort events; in fact, it's clear that he is making an enormous effort to be honest about them. It's just that he can't help but infuse them with his own point of view. Since David is a master of repression, his point of view is especially suspect.
Let's close with a question: why is David telling this story? In a way, of course, Giovanni's Room is a long confession. David confesses to his role in Giovanni's death, and he confesses to the fact that he led Hella directly into a dead-end marriage. But to whom is David confessing? We'll throw out the idea that David is actually confessing to himself, that he is telling this story to no one as much as himself (the reader is just granted a window). The point is that even the telling of the story is a part of David's search for himself. By trying to narrate his life in an honest way, David is trying to find out who he really is.