The vocabulary used by the characters in The Great Gatsby is indicative of the slang used in the 1920's by a younger generation breaking free from old, outmoded values and beliefs of the WWI generation. The "flapper generation" depicted in the novel came up with its own way of talking, much like today's generation has its own vocabulary for certain words or ideas. The 1920's was also a decade that saw jazz become popular music. Young people danced the Charleston; women wore shorter skirts and cut their long hair into bobs during this Jazz Age. It was a time of loose morals and having fun at all costs.
By using the diction of the 1920's, Fitzgerald places the novel in its social and historical context. This vocabulary keeps it true to the time period, and the novel will live on as a true artifact of the 1920's. The diction also makes the novel more believable as the characters represent a part of the culture in which they live.
Some of the 1920's words you will find in The Great Gatsby are:
Attaboy (well done)
Doll (attractive female)
Gold digger (woman who uses a man for his money)
Upchuck (throw up)
Whoopee (wild fun)
And, of course, Gatsby's favorite, "sport" (term of address)
The slang was so influential that we even use some of the words of the 1920's today!
If a writer like Fitzgerald, wants to be true to a time period or era like the Roaring 20's, he will use realistic language and actions (like the lavish parties Gatsby hosted) to make his literary work more believable and unforgettable.