What other method does Fitzgerald use to persuade the reader that Nick is credible?

Nick is a pretty credible character. He doesn't have any eccentricities and seems grounded. He was educated at the Ivy School Yale and fought in WW1. Nick isn't a character who needs anything. He seems to have enough money to settle down in West Egg. For much of the novel Nick is more or less a flat character. We can see life in West Egg through him. Nick observes situations without giving judgment. This allows the reader to trust Nick as a narrator and finally a character in the novel.