The climax and turning-point can be said to occur at the same time in The Great Gatsby. On what might be the hottest day of the summer, Gatsby accepts an invitation to visit with Tom and Daisy Buchanan (Chapter 7).
"They take a suite at the Plaza Hotel for mint juleps. Finally, Gatsby tells Tom that Daisy doesn't love her husband, and they confront one another, as Daisy falters" (eNotes).
Up to this point in the story, Daisy's rekindled affair (so to speak) with Gatsby has been kept a secret. Gatsby has even gone so far as to fire a majority of his household staff so that no gossip would get out about Daisy's visits to his house.
In this chapter, however, a confrontation ensues. Gatsby is brazen enough to accept Tom's invitation. Tom goads Gatsby and tries to draw him out. He succeeds.
There are quite a few interesting details to pull out of this section of the novel. Gatsby insists here that Daisy disavow her love for Tom and say that she never loved him. She cannot do it. Also, Tom badgers Gatsby and Gatsby, for the most part, remains calm and cool. He is forced to be direct in ways that he usually is not, but Gatsby stands in stark, cool contrast to Tom's hot-headedness.
It is in this area that another very interesting detail comes forward. Gatsby becomes the hero of the tale, in a subtle and rather indirect way, while Tom becomes the out-and-out villain.
When Gatsby is pressed to clear up the confusion about whether or not he is truly an "Oxford man," he offers a very believable and humble answer. Nick reacts with great relief. In his narration, Nick says of himself that "I wanted to get up and slap him on the back. I had one of those renewals of complete faith in him that I'd experienced before" (129). The honesty of spirit and genuine qualities in Gatsby here have risen above his many lines of deceit. It is Tom who is the bigger liar and the lesser man.
After this confrontation, Daisy kills Myrtle by running her over yet does not turn her in. Tom then leads a distraught Wilson to Gatsby, knowing both that Daisy was the one who killed Myrtle and that Wilson would probably kill Gatsby.
Tom's villainy is counter-poised with Gatsby's faith in Daisy and his faith in a certain romantic vision - the one that has effectively shaped his entire life.
Thus, the story's turning point and climax is also the place where the character's are revealed in their ultimate meaning or a light is shined on their truest interior selves.