Explain what Fitzgerald achieves by using Nick's point of view to tell Gatsby's story.

Numerous answers exist to your question about what the
author gains by creating Nick as the narrator of The Great Gatsby. 
He gains a narrator, a narrator who is a part of the action, and a narrator who on the
surface is an outsider who can objectively describe the shortcomings of the eastern
characters.  The author gains a persona who can contrast his midwestern ways with
the eastern ways of the other characters.  The author gains a persona who experiences
for the first time much of what he describes:  parties like Gatsby's, mansions like
Gatsby's, gangsters, etc. 

Perhaps more interestingly,
complexity and ambiguity are gained by the use of Nick as the narrator.  Fitzgerald
creates an unreliable narrator when he creates Nick.  Nick is not objective (he judges
Tom based on their acquaintance when they were in college, judges Jordan before he's
even introduced to her, etc.); thinks he's better than other people (you don't have to
try so hard not to judge others if you don't inherently think you are better than them);
and is not really an outsider (he knows Tom and Daisy before the present of the novel
opens, and is, in fact, Daisy's cousin). 

Nick's
personality and character are also questionable:  he dates the amoral Jordan, even
though he has judged her as such (as well judging her to be a snob), and he doesn't
notify the authorities that Daisy, his cousin, is the person who was driving the car
that kills Mrs. Wilson. 

The fact is that everything we
know about the characters in the novel is filtered through an unreliable narrator's
eyes.  That's complexity and ambiguity.  We don't have a story in the form of a novel,
we have a story as seen by and told by Nick.  He, therefore, is central to the
novel.