When are some instances in which the the last quote of The Great Gatsby played a role in earlier parts of the book?
The tremendous ending that Fitzgerald gives us in this book explicitly relates Gatsby and the plot of the novel to the wider historical setting of the new hope and possibilities of the American Dream as experienced by the first Dutch settlers as they land upon the "fresh, green breast of the new world." One of the interesting ways in which Fitzgerald himself relates the ending of his novel to earlier sections is through picking up the imagery of the colour green, and relating it to the light that Gatsby saw across the water, marking out the house of Tom and Daisy Buchanan. Note how the ending refers to this green light:
He had come a long way to this blue lawn, and his dream must have seemed so close that he could hardly fail to grasp it. He did not know that it was already behind him, somewhere back in that vast obscurity beyond the city, where the dark fields of the republic rolled on under the night.
Thus the ending is related to the symbol of the green artificial light that Nick spies Gatsby contemplating at the end of Chapter One and almost worshipping. Of course, the difference between the "fresh, green breast" of America and an artificial light seems to indicate the way in which Gatbsy's dream is devalued and debased, but at the same time it points towards the all-important theme of the American Dream in the novel, and how Gatbsy "believed in the green light, the orgastic future that year by year recedes before us." Even though his hope is misplaced, it is something that Nick is shown to admire.