The falling action in The Great Gatsby would have to be Gatsby's funeral. This is still part of the story, and we are still finding out things that we did not know before. For example, the reader finds out that Gatsby truly had no "real" friends. Nobody wanted to attend his funeral save his own father and the man that wore peculiar glasses (183). This shows the corruption that was rampant in society in the time of prohibition (I would argue that such corruption still exists today).
This would not be the dénouement, because the resolution ties up the loose ends. At this point in the story, however, the reader is still left guessing on many things.
"Why did Wilson go after Gatsby," "what do Tom and Daisy think about this whole ordeal," and "whatever happens to Nick?" are all good examples of questions that are left unanswered at this point in the story.
This would not be the dénouement, because the resolution ties up the loose ends. At this point in the story, however, the reader is still left guessing on many things.
"Why did Wilson go after Gatsby," "what do Tom and Daisy think about this whole ordeal," and "whatever happens to Nick?" are all good examples of questions that are left unanswered at this point in the story.