This sounds like it should be in the Literature Discussion Forum so you can get a collection of opinions and justifications for those thoughts.
Sordid is defined as meaning "marked by baseness or grossness; vile; dirty, filthy; wretched, squalid." Based on those definitions, I do not agree that The Great Gatsby is a sordid tale.
Certainly, Tom Buchanan is a thug and the villain of the story, but sordid doesn't appear to be an accurate description. All of the characters in the story are involved in deceiving themselves and each other in various ways - constructing life histories that bear minimal relationship with the truth, imagining amorous alliances that they have no intention of honoring or fulfilling - but the adultery and murder is based more upon delusions of fantasy, weakness of character, and refusal to accept the truth of their situations than upon sordid conspiracies.