You might want to think about re-visiting Chapter Three of this great novel, which is of course the first time that Nick goes to one of Gatbsy's parties. It is clear from the start that he does not describe himself as being one of the many partygoers that have descended on Gatsby's mansion. To start off with, Nick actually received an invitation, which the majority did not. Note how he describes himself and how is isolation and aloofness is established:
Dressed up in white flannels I went over to his lawn a little after seven, and wandered around rather ill at ease among swirls and eddies of people I didn't know--though here and there was a face I had noticed on the communting train...
As soon as I arrived I made an attempt to find my host, but the two or three people of whom I asked his whereabouts stared at me in such an amazed way, and denied so vehemently any knowledge of his movements, that I slunk off in the direction of the cocktail table--the only place in the garden where a single man could linger without looking purposeless and alone.
Note how Nick's outsider status is reinforced. He is "rather ill at ease" amongs the merriments and does not know anyone. He tries to act in what he feels is the socially prescribed way, seeking out the host, yet his attempts to do so are only met with amazement. In the end, he is left at the cocktail table where he can be alone and not feel so awkward. Such details help explain why Nick's report of the party comes across almost as a detached newspaper description of the events that occur and the people that he meets. He is detached and does not see himself as one of the partygoers, which gives him rather a privileged opportunity to watch the exploits of the rich and powerful from the sidelines.