Despite their being copious amounts of this at the party, what doesn't gatsby partake in? what could this show?

Gatsby does not partake in DRINKING

Gatsby met Dan Cody and what impacts Cody had on Gatsby's life.  One of the impacts that Cody had was that he, indirectly, made Gatsby not want to drink much. Gatsby doesn't want to drink too much because he doesn't want to be like Cody was when he drank, in the book Gatsby had had to be Cody's "jailer" at times.  That implies that Cody got out of control when he was drunk.  Gatsby's character does not seem to be one that would enjoy being out of control and I think this is why he hardly drank.