Hmmmmm, I would say that Nick learns a lot more about Gatsby in the car ride to lunch as opposed to the actual lunch itself. In fact, the only things Nick learns at lunch is that Wolfsheim also knows that Gatsby was educated at "Oggsford" and that Gatsby is close enough to Wolfsheim to know that he fixed the 1919 World Series.
The meat of the learning happens on the way to lunch while Gatsby and Nick are driving to town. Gatsby begins to reveal "God's truth" of his story (65). He was born to wealthy parents in San Francisco (which Gatsby calls the Middle West). Gatsby confirms that his parents have passed on, that he was educated at Oxford, and inherited his family's wealth. Nick's reaction here is interesting:
He looked at me sideways--and I knew why Jordan Baker had believed he was lying. He hurried the phrase "educated at Oxford," or swallowed it, or choked on it, as though it had bothered him before. And with this doubt, his whole statement fell to pieces, and I wondered if there wasn't something a little sinister about him, after all. (65)
In other words, Nick doesn't really believe a word that Gatsby is saying here. As readers, we already know that Gatsby lives in West Egg (not in East Egg), therefore he couldn't have "inherited" his money. We also know that San Francisco is never considered the Midwest. Other than these few facts, we have to simply observe Nick and decide for ourselves who we believe.
Now, when Gatsby explains further that he has "come into a great deal of money," Nick says that he "suspected that Gatsby was pulling my leg, but a glance at him convinced me otherwise" (66). Perhaps Gatsby is better at body language than wording. (However "come into" money does not necessarily mean "inherited" it, you know.)
Nick learns that Gatsby has lived in many of the cities of Europe, participating in extravagant activities such as ruby collecting and painting. Here, again, Nick's reaction is interesting:
With an effort I managed to restrain my incredulous laughter. The very phrases were worn so threadbare that they evoked no image except that of a turbaned "character" leaking sawdust at every pore as he pursued a tiger through Bois de Boulogne. (66)
Again, Nick realizes that this is all staged and doesn't believe a word of it. Gatsby, it seems, isn't a very good liar.
Despite all of this, Gatsby was hoping to die in World War I because of a sad experience he was hoping to forget. Gatsby later reveals that this "experience" through Jordan as the loss of Daisy. Gatsby fought bravely and was decorated for valor. With Gatsby's aggrandisement of "little Montenegro," Nick has a tiny change in feeling:
My incredulity was submerged in fascination now; it was like skimming hastily through a dozen magazines. (67)
What is important here is that when the proof becomes physical, Nick begins to believe Gatsby. For example, when Gatsby shows Nick his medal, Nick says "to my astonishment, the thing had an authentic look" (67). Further, Nick is totally taken in when he is given an Oxford photo of Gatsby holding a cricket bat. Nick says, "Then it was all true." Thus, Gatsby has gained a true believer all because of a medal and a photograph!
In fact, it is precisely this car ride and conversation where we, as readers, have to decide whether Nick is a reliable narrator or not. (Ironically, readers can prove that either way.)