Nick's father tells Nick, while Nick is young and impressionable, that he has some advantages over many other people whom he will meet in years to come, and that he shouldn't be quick to criticize a person until he understands their background. Nick's father wanted him to realize that his childhood had been tranquil, his education had been solid, his background of experiences led him to view the world with a positive and trusting outlook. The father wanted Nick to understand that others would approach life and the world from very different perspectives and that he shouldn't judge people without knowing their past.
Nick strives to follow this advice in his encounters with the people he meets during his time in New York. He tends to accept people as they present themselves and to believe their stories regarding their life's history until he is presented with information that contradicts the story. He believed that Jay Gatsby had grown up with wealthy parents and had worked as a successful businessman after completing his military service and time at Oxford until he started learning and seeing the proof that he was involved in drug trafficking.
Even when Nick has come to understand the true nature of his acquaintances, he doesn't judge them. He accepts the others for who and what they are. He perceived that Gatsby lied about himself because he couldn't face his truth, and accepted that situation as simply showing who Gatsby was and how he thought. It wasn't right or wrong, it was simply Gatsby.
Gatsby believed in the green light, the ogiastic future that year by year recedes before us. It eluded us then, but that's no matter-tomorrow we will run faster, stretch out our arms farther...