This is a good question, and a close reading of those four paragraphs really does help us gather up the themes of the novel. Notice how the paragraph begins with images of closed up places and a "shadowy glow" of the ferryboat (189). The images of light are throughout the novel, representing various kinds of beacons in the lives of the characters. The shuttered mansions are the "closing down" of Gatsby's dream.
Nick moves on then to a description of the "new world," America, and the dreams men dreamt as they discovered it, the last great frontier. The expanse of America promised endless opportunity and the hope of countless immigrants for a world in which class would make no difference. This is the world Gatsby aspired to, what we call the "American Dream."
Nick returns to the present and thinks of Gatsby, connecting in his mind, and for us, the "wonder" of Gatsby's dream, as that green light beckons him to Daisy, who represents his opportunity to transcend class and capture his "American Dream" (189). Gatsby believed that love and money could conquer anything, but he was wrong.
Nick concludes with the idea that this dream eludes all of us, but that we nevertheless keep running toward it, in our hopeless American optimism, although it is constantly moving away from us, lost in the past and in the inevitable nature of mankind, whether in the old world or the new, lost in our inability to live in the present, either dragging the past behind us or dreaming toward the future.