How are Gatsby's Parties emblematic of his era?

Because those were the so-called Roaring Twenties. That was the period of extreme wealth, both in the sense of money, and art and culture. Those were the "crazy years", of industrial growth, changes in lifestyle and cure, and of course, the blossoming of jazz music. In that way, the spirit of the whole era was crazy for parties, and thus, Gatsby's parties exemplified exactly that.