So far, the narrator, George and Myrtle are the only three characters we have met that aren't wealthy. Despite this, all of them unhappy. What does this imply about both the wealthy and non-wealthy groups?

Basically it implies that human nature is inherently selfish and superficial. THese people are unhappy because nothing real means anything to them. Nick could be meeting people with real interests that don't surround getting drunk and gossiping. George and especially myrtle have a relationship that was based on the illusion of something else. Sadly many people do that when they get married. See, Aslan is very wise!