Nick constructs himself first, before presenting Gatsby. He establishes himself as "the most honest person he's ever met," which is not saying much, considering the crowd he's with.
Gatsby is presented entirely in flashback. Whereas Nick's desires are vague (he likes Jordan, but he doesn't), Gatsby's desires are intensely focused (must have Daisy!). He is first seen by Nick, arms raised, worshiping her green light.
Gatsby becomes Nick's Byronic hero, his "bad boy" alter ego. Not gradually--right away. Gatsby is a passionate rebel, wounded by past love, and Nick digs this. Gatsby is mad, bad, and dangerous to know, and Nick wants to help him attain Daisy even though he knows it's a pipdream. To Nick, Gatsby is the embodiment of the American Dream. He is a true Horatio Alger story, as he goes from rags to riches. Certainly, Nick prefers Gatsby's romantic, self-made ideals over the pompous "good ole boy" network that allowed Tom to never have to work a day in his life.
Nick is the only one of the main characters who attends Gatsby's funeral, so there is some sentimentality involved. We actually get the boyhood story from his dad at the very end, which adds more Romantic flair to Gatsby's idealism.
The novel overall is perfection in organization; it is so well balanced. We don't meet Gatsby until Chapter 3 really, and by Chapter 8, he's dead. Chapter 5 and 6 are the turning points and climax respectively. So, it is amazing what Fitzgerald does in developing Gatsby within a relatively short novel.