There are many instances of ambiguity in Nick's dewscriptions of Gatsby. He says,
When I came back from the East last autumn I felt that I wanted the world to be in uniform and at a sort of moral attention forever; I wanted no more riotous excursions with privileged glimpses into the human heart. Only Gatsby...was exempt from my reaction -- Gatsby who represented everything for which I have an unaffected scorn.
Next, Nick begins his story. After serving in WWI Nick moves east from his Midwest roots to learn the bond business, settling on the island of West Egg, New York, "one of the strangest communities in North America". Nick reveals, however, that his story really begins on a June evening in 1922, when he drives over to East Egg, the wealthy of the twin islands, to have dinner with "two old friends whom I scarcely knew at all." Nick meets with an old college associate, Tom Buchanan, and his wife, Daisy, as well as Jordan Baker, an unexpected guest.
When the light conversation includes a brief reference to a man named Gatsby -- his next-door neighbor -- Nick's curiosity peaks. Tension mounts during dinner, however, when Tom leaves to answer a phone call, and Jordan reveals to Nick that it is Tom's mistress calling. Later, perhaps searching for sympathy in response to Tom's phone call during dinner, Daisy cynically tells Nick that she believes "everything's terrible". Though riveted by Daisy's voice while she speaks, Carraway finds her insincere, and leaves the Buchanan house feeling "confused and disgusted"
Upon arriving home Nick sees a silhouette emerge from the mansion next door, and assumes it is Gatsby. When Gatsby suddenly stretches his arms toward the water, Nick turns to see what he reaches for, but "distinguished nothing except a single green light, minute and far away, that might have been the end of a dock" When Nick looks back towards the silhouette, Gatsby has vanished.
This description highlights the ambiguity that Nick feels toward Gatsby almost a sort of an attraction, idealizing Gatsby. I think it is Fitzgerald's way of presenting the allure of Gatsby, as an almost magical figure, someone that represented an entire era, an escape from the horrors of the war, into irresponsible and reckless behavior.