I haven't read that book in a long time, but I think that when you are asked about someone's role in someone else's development, you need to show how person 1 influenced person 2 to change in some way. It's something to do with their minds and hearts, rather than external facts. Did 1 make 2 become more greedy, for instance? Did he do something awful that soured 2 on life? Did he love to drink, so made 2 his drinking partner, with the result that 2 ended up a hopeless alcoholic? Did his example change 2 into a better person, and if so, why?
So, think about what sort of person Gatsby might have been without Dan Cody in his life and the things Cody may have done that changed Gatsby's actions, purpose in life, view of life, etc. I recall that Gatsby ended up a suicide, didn't he? Did Cody have anything to do with that, if only to set Gatsby on the path that led him to giving up on life?