Explain how the last two sentences of this chapter continue the theme of Gatsby's dream.
"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."
In The Great Gatsby, Nick is the narrator and his version of events is what is revealed to the reader. His judgement that Gatsby is watching over nothing is his judgement, not Gatsby's. Nick's perception is accurate in terms of the big picture in the novel, and seen as so by readers, but he does not here speak for Gatsby.
The beauty of Gatsby's love is partly due to his devotion, his single-mindedness, his relentlessness. He doesn't give up even the next day when he spends it still hoping for a phone call from Daisy. This vigil is sacred to Gatsby, and he has not lost hope.
This is quintessential Gatsby, and his actions contribute to the theme of Gatsby's dream by keeping it alive.
A dream is most often not real. It is a figment of our imaginations. Sometimes we have goals that can turn into realities if we work really hard. Gatsby is working really hard for a dream, and it is just not in the cards for him.
The words eagerly and scrutiny make it feel like Gatsby is willing to work to make his dream a reality. But Nick can see reality as our narrator. We know he is seeing beyond the relationship Gatsby hopes to still have after the situation of this chapter and Nick sees things are not good. Nick leaves him watching over nothing, because it's over, this relationship. There is no reason for this affair to continue, and it really never does.
To me, the last two lines of Chapter 7 reveal how totally empty Gatsby's dream has been. He has been chasing something that seemed real to him. However, it is starting to be clear that there was no substance to the dream, nothing good about it.
Daisy has been Gatsby's dream all this time. He has seen her as the ideal woman and has believed that having her would make him happy. Now, the events of this chapter have shown that she has no morals to her, no real value.
Even so, Gatsby still idealizes her. He thinks she's sacred, but she really is (morally speaking) "nothing."
This is supposed to convey to us Fitzgerald's idea that the American dream has no real substance.