In Chapter VII of The Great Gatsby, Tom Buchanan reveals as much about Gatsby's wealth as Fitzgerald cares to divulge to the reader.
"Who are you, anyhow?" broke out Tom. "You're one of that bunch that hangs around with Meyer Wolfsheim--that much I happen to know. I've made a little investigation into your affairs--and I'll carry it further to-morrow."
"You can suit yourself about that, old sport," said Gatsby steadily.
"I found out what your 'drug-stores' were." He turned to us and spoke rapidly. "He and this Wolfsheim bought up a lot of side-street drug-stores here and in Chicago and sold grain alcohol over the counter. That's one of his little stunts. I picked him for a bootlegger the first time I saw him, and I wasn't far wrong."
Gatsby has probably made most of his money from selling liquor. No doubt he has been in other illegal ventures with Meyer Wolfsheim, who uses him as a front-man because of his good looks and good refined manners. Fitzgerald, however, does not want to make Gatsby seem any worse than necessary for the purposes of his story. Once Tom Buchanan exposes Gatsby as a criminal in front of Daisy, Gatsby has lost her irrevocably. Tom has managed to hold her even though she has said she doesn't love him anymore and he apparently only values her as a trophy wife. Fitzgerald wanted to make Gatsby look as good as possible without denying that he is a man who is involved in multiple criminal enterprises and may have used violence or had others do violent acts for him, including possibly murdering rival gangsters.
No one knows for sure how Gatsby got his money, but there are several educated guesses and rumors.
The most prevelant rumor is that Gatsby got his money through bootlegging (illegal sale of alcohol), which he did through his ties to the mob.
Daisy tells Tom at one point, that Gatsby owned a chain of drug stores, and that is how he got rich. He might have indeed owned some pharmacies, but if he did there was most likely some illegal activitiy connected with them.
Gatsby intimated also that he came from a weathy family "out west". But
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:
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.