Prohibition, of course, was a law banning the sale and consumption of alcohol. Gatsby throws large parties with extravagant bars. Everyone knows that liquor is being served, yet legal enforcement is never a factor. The elite flaunt their ability to serve and consume liquor, as if they are above such petty laws.

I think in a couple of ways.  First, there are great rumors about where Gatsby first aquired his money.  Bootlegging is the most popular suspicion.

 More importantly, for a time in American history where alcohol was deemed unsafe and unfit for social consumption, don't you think an awful lot of these events occur with the presence of booze?  All of Gatsby's parties are centered around the idea of free food and drink.  Nick admits to being too drunk at Tom and Myrtle's apartment to really remember what happened.  Everyone was drinking at the Hotel the day of Myrtle's death.  And at many of these social drinking outings, Fitzgerald has made it clear that the use of alcohol makes for a "proper" atmosphere.  How much fun would the various people at Gatsby's parties have been if they were sober?  Further, Gatsby throwing these huge, alcohol infused parties was his way of inviting Daisy's crowd (and her) to his place.  Would Daisy's crowd (and she) have been present without the promise of free booze?