The effects of Prohibition about The Great Gatsby:

1.  Gatsby most likely would not have made his millions or been able to pursue Daisy if the novel were not set during Prohibition.  Because of the illegality of alcohol in the U.S., Gatsby's "drug store" business (i.e., bootlegging ring) flourished. Prohibition stimulated the growth of the nouveau riche, such as Gatsby and some of his party-goers, and engendered the mingling of old money society with those who would have once been "commoners."

2.  Appearance, gestures, and actions--the flappers and party girls (such as Jordan) felt freer during Prohibition.  Of course, this relates to women gaining suffrage, but Prohibition also allowed for women to be freer with their dress and actions.  If they didn't feel like following the rigid standards of high society, they could wear short party dresses, cut their hair, and drink (illegally) to establish their "rebellion."