Surely this is impossible....

In Chapter 8 Nick gives an incredibly detailed resume of Gatsby's trip to Louisville when Daisy and Tom were on honeymoon. This is not in speech marks, so are we expected to believe that Nick has remembered every tiny detail? We're talking about subtle nuances of emotion which only had meaning for someone being there at the time. This stands out as being extremely unlikely- in fact impossible. Has Nick invented this- is this a fiction within a fiction? Does this not throw into doubt the veracity of the rest of his narration? What do we trust?