How did Jay Gatsby get all of his money in The Great Gatsby?

Like all good novels, The Great
Gatsby contains elements of mystery. This is the case with Jay Gatsby's
fortune. He throws elaborate parties and impresses everyone, but no one really knows
where this man came from and how he made his money. What makes Gatsby's wealth even more
elusive is that he never says how he made his money. When someone asked him, he simply
said that was his business. Here is the text:

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I think he hardly knew what he was saying, for
when I asked him what business he was in he answered ‘That’s my affair,’ before he
realized that it wasn’t the appropriate
reply.

That said the book
implies that Gatsby made his fortune in an unsavory
way. 

At one part of the book, people believe that Gatsby
made his money from bootlegging, that is, the sale of alcohol. This might sound odd, but
from an historical point of view, the sale of alcohol was profitable, owing to the
prohibition movement. From this perspective, the sale of alcohol is tantamount to the
sale of drugs today.

More concretely, Daisy tells Tom that
Gatsby owned a chain of drug stores. It was presumably here that Gatsby distributed
alcohol.