This passage comes when Gatsby is telling Nick about his love for Daisy, and how he never was able to recapture the first glorious feelings of love he had for her after her marriage to Tom:
He stretched out his hand desperately as if to snatch only a wisp of air, to save a fragment of the spot that she had made lovely for him. But it was all going by too fast now for his blurred eyes and he knew that he had lost that part of it, the freshest and the best, forever.
(Fitzgerald, The Great Gatsby, ebooks.adelaide.edu)
Returning from France, Gatsby visits Louisville, where he had courted Daisy and fallen in love, but the place seems empty and barren without her presence. Knowing that she is married and therefore seemingly out of his reach, the visit is more of a desperate attempt to relive his feelings. When he leaves the city by train, he "stretches out his hand" in a metaphorically physical grab for happiness, but he knows that the innocence in their early relationship is gone forever.
This passage, narrated in the third-person by Nick, shows some of the poetic pain that Nick feels in sympathy with Gatsby, and idealizes Gatsby and Daisy's love while acknowledging that events have moved on, leaving Gatsby behind.