In the book, Jay Gatsby is constantly calling people "old sport." So I guess you would say that that is his favorite expression. It is something that he does to show that he is an "Oxford man," even though he never actually attended school there.
Gatsby had a good reason to believe that he was going to inherit Dan Cody's money. But he only ended up with a very little bit of it because Ella Kaye got it. We know nothing about her except that she came on board the yacht in Boston and a week later, Cody was dead. She used her lawyers to be sure that Gatsby would never get very much of Cody's money -- just $25,000 out of the millions that he had.