In writing about this passage, you will want to explain the meaning in the moment (the topical meaning) and also explain the significance of Nick's statement (contextual meaning). This gives you two sub-topics for an essay that can be further broken down into sections.
At this point in the story, Daisy has killed Myrtle with a car and retreated behind closed doors and behind her husband, Tom. Gatsby sits by the pool at his mansion waiting for a call from Daisy.
...unaware that Daisy and Tom are already packing their bags for an escape abroad, he waits for Daisy’s call...
He has yet to give up hope. He still believes that his dream of marrying Daisy can come true. Nick, of course, realizes that this will not happen.
However, it is Gatsby's penchant for hope and self-assurance that creates the eventual respect and affection that Nick feels for Gatsby. Despite everything that has happened, Gatsby is still innocent enough to wait hopefully for the phone to ring.
This hope is, at least partly, what sets Gatsby apart from the cynical crowd of Jordan, Daisy, Tom and the rest of the "whole rotten bunch". Where they choose to shrink from responsibility and hide out, Gatsby remains exposed, outdoors, with his heart still open to the possibilities of success (and exposed therefore also to the possibilities of defeat).