What does Gatsby reveal to Nick in the car on the way to the city?

I believe the scene you are referring to is when Gatsby picks Nick up and drives him into the city with him to get lunch. Up to this point, Nick knows very little about Gatsby, and he doesn't have an impression of his life being very interesting.

I had talked with him perhaps half a dozen times in the past month and found, to my disappointment, that he had little to say. So my first impression, that he was a person of some undefined consequence, had gradually faded he had become simply the proprietor of an elaborate roadhouse next door.

So, Nick is very surprised when Gatsby launches into a history of his life. He tells him that he was born in San Francisco, came to age in America, and then received an education at the University of Oxford. Following this, his family died and he received inheritance from them. Then he:

lived like a young rajah in all the capitals of Europe—Paris, Venice, Rome—collecting jewels, chiefly rubies, hunting big game, painting a little, things for [himself] only, and trying to forget something very sad that had happened to [him] long ago.

He ends his story by telling of his time in the war, saying he was promoted to Major by the end of his service.
Nick initially does not believe any aspect of the story, and is only able to hold back his "incredulous laughter" "with an effort." However, Gatsby produces proof for him, leading him to conclude that "it was all true." Of course, it was not all true: Gatsby was born in North Dakota, only attended Oxford for a short period of time, and likely made all his money from bootlegging rather than from inheritance and gaudy jewel collecting. This scene exposes some parts of Gatsby's history, but it also exposes him as a liar and unable to open up to Nick truthfully.