This is a good question. There are many places in the text that speak of Gatsby's parties. Chapter three is a good place to start. Right from the beginning the parties are set up. People not only come by cars, but also boats and planes. It smells of extravagance.
There was music from my neighbor’s house through the summer nights. In his blue gardens men and girls came and went like moths among the whisperings and the champagne and the stars. At high tide in the afternoon I watched his guests diving from the tower of his raft or taking the sun on the hot sand of his beach while his two motor-boats slit the waters of the Sound, drawing aquaplanes over cataracts of foam.
Here is a description of the food at the parties, and once again we can see the decadence. Excess is the point.
At least once a fortnight a corps of caterers came down with several hundred feet of canvas and enough colored lights to make a Christmas tree of Gatsby’s enormous garden. On buffet tables, garnished with glistening hors d’oeuvre, spiced baked hams crowded against salads of harlequin designs and pastry pigs and turkeys bewitched to a dark gold. In the main hall a bar with a real brass rail was set up, and stocked with gins and liquors and with cordials so long forgotten that most of his female guests were too young to know one from another.
Here is a description of the people:
Laughter is easier, minute by minute, spilled with prodigality, tipped out at a cheerful word. The groups change more swiftly, swell with new arrivals, dissolve and form in the same breath—already there are wanderers, confident girls who weave here and there among the stouter and more stable, become for a sharp, joyous moment the center of a group and then excited with triumph glide on through the sea- change of faces and voices and color under the constantly changing light.
The sadness of all of this is, of course, that it is completely shallow and superficial.