1) The truth that Gatsby tells Nick in this chapter is a wide and varied history of himself, about how he had wealthy Midwestern parents from SF , was educated at Oxford, collected jewels in Europe, hunted big game , and was awarded medals in WW1 .
2) The truth that Jordan tells Nick is that Gatsby is in love with Daisy Buchanan and that Gatsby bought his mansion in West Egg Solely to be near Daisy.
I don't believe Gatsby because as he unfolds his story he gives the first strong indication that he isn't quite what he presents himself to be and he's not a good liar.
