Where the green light had once stood for Gatsby's dream and his hope to achieve the ideal marriage with Daisy, at the end of the novel this is no longer possible.
As hope is an essential component of Gatsby's character, the light that represents his hope attains a "colossal significance" for Gatsby and for the novel. Nick repeatedly aligns Gatsby with this hope and a curiously naive idealism, even in his first descriptions of Gatsby.
"If personality is an unbroken series of successful gestures, then there was something gorgeous about him, some heightened sensitivity to the promises of life.… [Gatsby had] an extraordinary gift for hope, a romantic readiness such as I have never found in any other person and which it is not likely I shall ever find again."
Gatsby's view of the green light across the bay from his house is one of enchantment because Gatsby is thoroughly taken up with the idea of fulfilling his ideals and his dreams, which are focused on marrying Daisy. Her house is on the other side of the bay, near the light. When she retreats into that house after running down Myrtle in a car accident, Daisy goes beyond Gatbsy's reach. The enchantment ends.
Daisy has given up on marrying Gatsby. She seems to give up during the confrontation episode where Tom and Gatsby each make claims on Daisy's love.
In this episode Gatsby demands that Daisy say she never loved Tom (which she refuses to say) and Tom reveals a number of tawdry rumors about Gatsby's underworld connections. Daisy is disappointed and upset by this episode and Gatsby's hope to win her over end.