The more compelling argument is that the climax of The Great Gatsby comes in the confrontation episode, when Tom, Gatsby, Daisy and Nick are all together at the same time.
This scene brings the major story-lines of the novel together and the tensions which had been building thus far gain full expression and are brought out into the open.
Gatsby's quest ends in this episode when Daisy refuses to say that she never loved Tom. Daisy's dilemma comes to an end here as well as she sees for the first time the true nature of her choice (to be with Gatbsy and render her entire past meaningless or to remain with Tom and give up on the dream of high romance).
We can argue that the climax is carried through to the car accident. It is the accident which kills Myrtle that brings an end to another major story-line, the affair between Tom and Myrtle.
Not only is the accident a high dramatic point, but there is also a noticeable shift in tone as the narration turns to resolution immediately afterwards.
The drama is over as is the sense that anything more meaningful will happen. Though Gatsby's death is still to come at this point, we can argue that his death is part of the novel's resolution, a part of the aftermath of the summer's events. (This notion is reinforced by a number of details in the narrative, including the idea that the leaves will fall soon and the pool is going to be drained.) The death really changes nothing and so is not significant as a plot point.