The narrative posture in the novel is one of intimate conversation. Fitzgerald has Nick recount the story of his summer on the east coast as if speaking to a friend or family member, years after the events take place.
When Nick utters his final words in the novel, he is continuing to speak to this silent figure. Of course, the audience (the reader) is the "silent figure", invited into Nick's confidence and privy to Nick's various boasts, confessions, and observations.
The story is told from a retrospective position and so the idea of being "borne ceaselessly into the past" is appropriate for a narrative that recounts so detailed an remembrance of a time that is now years in the past.
Gatsby believed in the green light, the orgastic future that year by year recedes before us. It eluded us then, but that's no matter--tomorrow we will run faster, stretch out our arms farther. . . . And one fine morning----
So we beat on, boats against the current, borne back ceaselessly into the past.
to everyone, he is talking about the green light, the green light for Gatsby symbolises the unattainable dream. Gatsby has spent his whole life longing for something better.. money, success, acceptance and Daisy. And when Nick says this at the end of the book, he connects the green light to all people: we all have dreams, no matter how unattainable they are, we dont give up, like gatsby.