In Chapter 5 of The Great Gatsby, Gatsby offers to help Nick "pick up a nice bit of money." Gatsby, who is fabulously and mysteriously wealthy, tells Nick that he has "a little business on the side, a sort of sideline," to which he would like to give Nick an opportunity to become involved in.
The reason that Gatsby makes this offer to Nick is quite simple: it is payment "for a service to be rendered." The service is that Nick has agreed to arrange a meeting between Daisy (Tom's wife) and Gatsby, who had been in love some years before.
Gatsby is, in fact, obsessed about renewing his relationship with Daisy. He purchased his magnificent house because it sits just across the bay from Daisy's house, and the elaborate parties he hosts are intended to attract Daisy.