Narrator Scott Shepherd, who plays Nick Carraway, concentrates on reading “The Great Gatsby” aloud as the characters in the book come to life with a roaring party around him. “Gatz,” an Elevator Repair Service production running at the American Repertory Theater, sets the popular Fitzgerald novel in a mundane office that gradually morphs into the tragic world of “Gatsby.” Poignant and arresting, “Gatz” will show at the A.R.T. through February 7.