The Great Gatsby ends unexpectedly. Gatsby is killed, his funeral is empty, save for his father, Nick and that owl eyed man. Not even Daisy shows up. This exemplifies Gatsby's loneliness.
For 5 years he has been chasing the green light across the water. Always trying to recreate what it was like in the past, but in the time it took him to buy the car that'd allow him to join the race for Daisy's love, the past had already gone by, the tide had left the shore, and all Gatsby could do was try to fight the current of the past, while never looking to the future. When Nick first saw Gatsby, he was reaching for the green light across the bay. This light seemed unreachable, and indeed it was. Gatsby couldn't reach Daisy, couldn't reach that hopeful green light.
Nick ends up leaving the east, going back to the west. While he sees Tom, and it's revealed Tom told Wilson it was Gatsby's car, Nick doesn't tell him who was really driving. Nick also talks to Jordan, finally ending his relationship with her. Nick takes one last look at that "failure of a house." Still empty, and seemingly forgotten by the world. Nick leaves New York and that haunting past behind him, once and for all.