Nick mentions that he has just come back from the war. This is World War I. Nick refers to it as the "Teutonic migration known as the Great War." He sarcastically notes that he "enjoyed" his time in the war so much that he came back "restless." One might conclude that Nick could be suffering from post-traumatic stress disorder. But from his behavior and observations throughout the rest of the novel, this doesn't seem to fit his definition of "restless."
It seems that Nick is making a comparison between the East and the war itself. For Nick, his home (the Middle West) has associations of nostalgia and humble lifestyles. The East, on the other hand, has associations of corruption. Nick moves East out of a general restlessness, being 30 years old and just coming from a war. But he also notes this restlessness to set up a comparison between the corruption of the East and the war, and to differentiate the East from the more humble Middle West of his childhood.