Tom has made an investigation into Gatsby's occupation by the time the reader arrives at the middle of chapter 7. This investigation has revealed to Tom that Gatsby could not have spent four years at Oxford, and that Gatsby's services are less than moral.
Tom reveals that Gatsby is connected to Meyer Wolfsheim for work purposes and that Wolfsheim is a swindler. The most specific Tom gets in describing Gatbsy's work occurs in these words:
I found out what your 'drugstores' were.... 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 asks Tom what's so wrong with that, thereby admitting that this is indeed part of his line of work. Tom later insists that Gatbsy is up to something even more immoral, so much so that Tom's friend won't reveal the secret to him.