At the end of Chapter VII, Tom, Daisy, Jordan, and Nick have returned to the Buchanan's house in East Egg after Myrtle Wilson had been killed on the road. Nick has been invited inside, but has refused. As he is walking down the drive to wait for a taxi, he encounters Gatsby who seems unconcerned about Myrtle's death but terribly concerned about Daisy's welfare. He tells Nick that he will wait to make sure that Tom does not "bother" Daisy and that he will wait and watch all night, if necessary. Nick recalls:
He put his hands in his coat pockets and turned back eagerly to his scrutiny of the house, as though my presence marred the sacredness of the vigil. So I walked away and left him standing there in the moonlight--watching over nothing.
The scene is reminiscent of the first time Nick had seen Gatsby. The moment is narrated at the end of Chapter I when Nick sits on his lawn and observes a figure on the adjacent lawn:
. . . fifty feet away a figure had emerged from the shadow of my neighbor's mansion and was standing with his hands in his pockets regarding the silver pepper of stars . . . he stretched out his arms toward the dark water in a curious way, and far as I was from him I could have sworn he was trembling. Involuntarily I glanced seaward--and distinguished nothing except a single green light, minute and far away, that might have been the end of a dock. When I looked once more for Gatsby he had vanished . . . .
The green light is the light at the end of the Buchanan's dock across the bay; as in the passage from Chapter VII, Gatsby stands in the dark and looks toward Daisy's house.
The difference in the two scenes is one of timing and purpose. When Gatsby looks toward Daisy's house the first time, he longs to meet her again, to have her back in his life; it is a time of romantic possibility for Gatsby. He watches her house to feel close to her.
When he stands vigil and watches the Buchanan house in Chapter VII, his romance with Daisy is over, although he cannot acknowledge it. While he stands alone in the darkness, Tom and Daisy are inside, sharing a cold supper. Before leaving Gatsby there to keep watch, Nick had observed through the kitchen window "an unmistakable air of natural intimacy" between Daisy and her husband. Gatsby's dream will never be realized; romantic possibilities are ended. He watches over Daisy's house to protect her from Tom, not realizing that Tom and Daisy are together and will remain together. As Nick said, Gatsby watches over nothing.
He stand out looking at her house in the start of the story with a drive because he loves her, but this time he looks out thinking that their love wont be what he thought it would.