Nick keeps track of celebirties in Gatsby´s parties just to try to get some more and real information about him. Althoguh Gatby tells Nick his story, he does not honeslty belive in it as there are many details and events that are confussing.
Besides, we can say Nicke did that as any regular person would do when encountering famous people. Nick belogs to middle to low class in society and this high-class events and contacts Gatsby offered was an attraction and perhaps an opportunity to himself.
