what expression does gatsby use repeatedly in his speech in referring to the people directly in chapter 3 and why?
Gatsby refers to a number of people as "old sport" which would suggest both a geniality and a familiarity that he didn't seem to have. He refers to Nick this way though they've only just met and the phrase tends to lend an air of artificiality to his interactions with people, something that may in fact be due to his singular interest in Daisy.
Of course it may also be an affect he assumed in order to better play the part of the man with more money than god who speaks to everyone that way. Particularly because of its repetition, it certainly does little to differentiate between people he truly cares about and those who are just hanging on at his party.