Because these two chapters contain the most dramatic events in the novel, and thereby hold the reader's attention, chapters 7 and 8 do seem to fit the definition of progression d' effet, that is the effect of illustrating an accelerated progression of events and revealing insightful information.
Chapter 7 opens with a clue that worlds have at least come close to colliding (therefore creating the opportunity for potentially dramatic events and revelations) when Gatsby dismisses his servants to avoid calling attention to Daisy's visits. Although it is in Chapter 6 that Nick relates the story of Gatz to Gatsby (in a flashback) it is in Chapter 8 that Gatsby relates his story to Nick. So, the unfolding of this information about Gatsby occurs, within the world of the story, in Chapter 8.
The other sense of progression d' effet is that all the author's techniques and words contribute to this progression of effect; that is to say that each element in the story (or in these two chapters) must contribute to that idea of a revelatory series of events, prose, and/or dialogue which of course must hold the reader's attention. Fitzgerald does do this quite well since these are the two chapters where characters' secrets are revealed to each other and the result is, as Nick describes it, a kind of "holocaust."
In Chapter 7, the mood of the get-together (Tom, Daisy, Nick, Jordan) is intense; the tension is amplified by the physical heat of the weather. All of these techniques contribute to the increasing tension which results in the hit and run. In Chapter 8, Gatsby fills Nick in on the details of his past while they (and the reader) wait to see how and if things will work out with Daisy. The wait is irrelevant as the narrative shifts to Wilson's angst and his assumption (hinted at by Tom) that Gatsby was the one driving the car. Chapter 8 concludes with the finale of these secret worlds colliding leading to Gatsby's murder and Wilson's suicide. Like the rising tension in Chapter 7, Wilson's movements also suggest something dramatic is about to occur:
Thus far there was no difficulty in accounting for his time—there were boys who had seen a man “acting sort of crazy,” and motorists at whom he stared oddly from the side of the road.
Fitzgerald does use progression d' effet successfully, specifically in these two chapters because these are the sections of the novel where/when the story is carried forward at an accelerated rate with more dramatic scenes and revelations: where the worlds collide.