According to Tom, how did Gatsby get his money?

From the text:

“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.”