F. Scott Fitzgerald uses many dashes throughout the novel The Great Gatsby. In the opening paragraph of chapter 7, a dash is used twice (in three sentences).
A dash, in literature, is used to show the reader that the information following it is important. Essentially, the dash is used to emphasize the information which follows it.
At the end of chapter seven, one finds the following dash:
So I walked away and left him standing there in the moonlight — watching over nothing.
Here, Nick Carraway has concluded that Daisy had been driving the "death car." Jay Gatsby denies this and states that he had been the one driving the car. Nick, in going to get into his waiting taxi, leaves Gatsby "standing in the moonlight."
Here, the dash emphasizes the fact that Gatsby was not watching over anything as Nick leaves him standing alone. Fitzgerald wants readers to infer that Gatsby is not watching over anything, especially Daisy (as alluded to when Nick states that Daisy had driven the car).