I'm not exactly sure what you mean by Gatsby's value in the party scene of The Great Gatsby. I don't know if you mean what is revealed about him, how other characters value him, or how valuable he is to the party. I'll do what I can.
Gatsby provides everything for the party, and it is lavish in the extreme. The party reveals the Jazz Age that Gatsby is a part of. Most of the people are shallow and hedonistic (they are obsessed with pleasure), as well as terribly reckless. Gatsby does not take much of a part in the goings-on, and he does not drink the illegal alcohol. He does provide it, however.
The party demonstrates the lengths Gatsby will go to for a chance at meeting Daisy. He throws the party, and his other parties, as well, hoping that Daisy will happen to come to one by chance and he will get to see her again.
Gatsby's aloofness leads to rumors about him. The guest wonder about his job, his role in the war, and his past.
The party contributes a great deal to the novel. I hope I hit on some of the points you need.