F.Scott Fitzgerald shows off his wealth to the reader. You can see this in "On week-ends his Rolls-Royce became an emnibus" this implies that Gatsby has enough money to let expensive cars (Rolls-Royce being known for being a signifying wealth) being used for a casual perpose.
Later in the text we are shown that the night is going well " alive with chatter and laughter" this shows us that everyone is enjoying their night and also getting drunk. However, the verb chatter implies what they are saying to eachother means nothing.
As the text develops the writer says " spilled with prodigality" this shows the rerader that the laughter has overflowed, this may be because of the amount of drink they have engolfed.