In addition to the euphemisms of the Jewish underworld boss, Meyer Wolfscheim, there are others that are, perhaps, more subtle. For instance, Nick prides himself in Chapter Three for being "one of the few honest people" that he has known. In this same chapter, Nick proclaims that towards Jordan Baker he does not feel love, but "a sort of tender curiosity." When he learns of the incident that reached the paper in the "proportions of a scandal," he remarks that Jordan is "incurably dishonest," and about this, he is "casually sorry" and then, he says, "I forgot."
In Chapter Four, Nick's apparent penchant for euphemism continues as he narrates that he had written down the names of those who came to Gatsby's house the one summer. Although it is disintegrating, Nick states, he is still able to read the "grey names" of those who took advantage of Gatsby's
hospitality and paid him the subtle tribute of knowing nothing whatever about him.
In Chapter Four, as Gatsby alludes to the battle at Montenegro. a small Italian kingdom that came under Fascist control; the conflict with Montenegro became vicious and chaotic; a guerilla war began that cost tens of thousands of civilians their lives. But, as Nick listens to the account of Gatsby, he states that Gatsby's smile "sympathized with the brave struggles of the Montenegrin people":
It appreciated full the chain of national circumstances which had elicited this tribute from Montenegro's warm little heart. My incredulity was submerged in fascination now; it was like skimming hastily through a dozen magazines.
Fitzgerald's prolific employment of euphemisms in Nick's narration indicates the superficiality of the characters who populate Fitzgerald's tableau of the Jazz Age, a people caught from within in illusion.
A euphemism, of course, is a way of saying something in better, less offensive or blunt terms. For example, when someone has died, we often say they've "passed on" instead of the harsher, more realistic term. Short people might prefer to be called "vertically challenged." If someone is trying to make the job of garbage collector more glamorous, we might say "sanitation engineer" instead. If politicians or statisticians are talking about murder, they usually use the less emotionally charged term "homicide." What all of these euphemism have in common, though, is that they--at least to some degree--serve to hide the original concepts of death, shortness, garbageman, and murder. They are not the clearest and purest form of language, then, because they disguise something about the original word.
That being said, the one I like most and thought of right away when I saw your question was "business gonnegtion" (obviously it's a certain New York pronunciation of connection). It's located in chapter 4 (p. 75 in my book) where Gatsby introduces Nick to Meyer Wolfsheim. Wolfsheim is a shady, underworld character associated with (perhaps responsible for) the 1919 World Series "fix." When Wolfsheim syays to Nick "'I understand you're looking for a business gonnegtion,'" he's really inviting Nick to join him in his illegal business dealings.
How about Jordan referring to Myrtle as "some woman" instead of a mistress (chapter 1 p. 19).
I'm certain there are many more, but this should help get you started.