The green light at the end of Daisy Buchanan's dock symbolizes Jay Gatsby's idolatrous pursuit of Daisy and the American dream of wealth and material possessions.
Gatsby's self-definition and identity cannot be separated from his crass and showy opulence -- themes accentuated by Baz Luhrmann's special effects in which he moves cinematically from the Gatsby mansion to the Green Light across the Bay at Tom and Daisy Buchanan's mansion.
The American dream when it is focused narrowly on the acquisition of material wealth, status and power becomes a self-destructive nightmare. Consumerism is an idol. It can never truly engage the depths of the human heart and soul. It only stunts them.
We become in the words of T.S. Eliot hollow men and hollow women. "We barter the infinite for an infinite series of finite things." (Mother Maria Arioli, OSB)
As St. Augustine says so simply and profoundly: "Our Hearts are Restless until they rest in thee."
Contrast the green light in The Great Gatsby symbolizing the American Dream turned nightmare, with the red tabernacle light at the heart of the Marchmain family Chapel in Evelyn Waugh's Brideshead Revisited. (You may remember the outstanding PBS series in the early 1980s).
That red light symbolizes the real and true presence of Jesus Christ in the Holy Eucharist and in the world.
The characters in Brideshead Revisited experience many of the human weaknesses and sins of the characters in The Great Gatsby.
The difference is that the objective truth of Christ's presence in the world in the Holy Eucharist leads them from the path of self-destruction and sin to the path of conversion, virtue, grace and holiness.
As JRR Tolkien wrote to his son at the end of his life: "I put before you the one great thing to love on earth: the Body and Blood of Jesus Christ. There you will find true romance, true honor, true glory and the true way of all your loves upon earth."