In The Great Gatsby when Myrtle's apartment is being described, what rhetorical devices does Fitzgerald use to support the old/new money theme?
Chapter 2, as you indicated, includes the episode at Myrtle's apartment. She has a small party that results in Tom breaking her nose and Nick waking up drunk beside Mr. McKee. Tom is the representation of old money in this scene, but new money is not truly represented here. Instead Fitzgerald shows us social climbers--Myrtle despises her humble origins, thinks she's married beneath her, and clings to Tom as her ticket to real money. However we soon learn that Tom is merely using Myrtle; he has no intention of marrying. He puts her up in the apartment because he can.
As far as rhetorical devices to show the differences between the Haves and Have-nots, Fitzgerald uses Nick, the narrator, to describe the action in a more or less objective fashion. As Nick describes himself much like a "casual watching" looking in on the action through a window:
I was within and without, simultaneously enchanted and repelled by the inexhaustible variety of life.
He describes the various guests that arrive--Catherine and Mr. MeKee, both of whom are depicted somewhat comically. Catherine has a "sticky bob of red hair" and her "eyebrows had been plucked and then drawn on again." Mr. McKee was a "pale feminine man from the flat below" with a spot of shaving cream on his cheek. Neither of these guests seem to have much money. McKee is a photographer with a bland personality and Catherine, a beautiful woman, jangles cheap jewelry.
We see a contrast in the way Myrtle acted at her house and the way she acts in the apartment. Her new dress changes her from being filled with vitality to having an "impressive hauteur."
Also included is somewhat vapid dialogue with guests talking but not truly connecting. Much of the conversation is pretentious with Mrytle pretending to have more money than she does. But what is clear is that Myrtle despises her husband, and her main reason for despising him is so trivial that it is laughable: the suit he was married in was a borrowed suit.