At the end of chapter 7, Gatsby is waiting outside as he believes Daisy will leave her husband to run with him, so he is waiting for her to give him a sign. However, Nick, who can see the house from a different angle, can observe Daisy and Tom sorting out their problems instead of fighting. He knows this means that Daisy will stay but doesn't tell Gatsby as he watches him "...watching over nothing". This expression lets the reader know that Gatsby's hopes and expectations will not be fulfilled.
