Firstly, I would discourage simplistic readings of the other characters. Gatsby uses the Buchanans, who are, indeed, unappealing people, to critique the American Dream. The Buchanans embody the social prestige of "old money" that Gatsby seeks to gain. Though Gatsby has earned a lot of money through bootlegging (a business made successful due to demands of alcohol from people like the Buchanans), he can never gain entry into the world of the Buchanans.
Arguably, each character in the book is an archetype from American life: Tom Buchanan is an entitled racist; Daisy is a wealthy, passive woman from the South who has been taught to believe that youth and beauty can protect her from unhappiness; Myrtle Wilson is a social climber who allows rich Tom Buchanan to abuse her so that she can be near him; Myrtle's husband, George, is a poor man who is taken advantage of by those who have more than he, living in a world he only partially comprehends.
Gatsby, as Nick Carraway narrates at the end of the novel, embodies the hope that existed in New York's first Dutch settlers. He started as a poor boy, probably of Jewish extraction (his name was James Gatz) who exhibited an unusual level of discipline at a young age and used that to become a success. He embodies the American value of hard work as a path to success. He is always striving—an aspect of his character that is illustrated by his reaching, literally, toward the green light that marks the shore of East Egg where Daisy lives.
Having written all of this, the question of whether Gatsby deserves sympathy is a subjective one. Most would agree that the novel's outcome is sad, and Fitzgerald offers no moral comeuppance for the Buchanans. However, Gatsby is a snob who distances himself from his humble origins and embraces materialism. He probably loves Daisy, but it is unclear because he seems more focused on her as a thing to win, a way to enter an exclusive social class.
Fitzgerald uses The Great Gatsby to critique the American Dream and to find fault with the notion that everyone can succeed. Gatsby, for all of his wealth and notoriety, never felt successful and never got what he wanted. The Buchanans, though in possession of the prestige that Gatsby desired, are miserable people. The American Dream is a creation of the imagination, something elusive and always just out of reach, like the green light on a distant shore.