The story from which this movie draws its plot is an average one.
Actually, the story is what took it from being an initial flop that only sold a few thousand copies to required reading in American Lit classes.
It's a periodic story of the American Dream and how that dream can lead to ruin if you're not careful. If anything, it is a story that sparked the trend which makes it, now, seem average in comparison.
However, and as you stated, the story became a part of the American literary canon because of Fitzgerald's eloquent writing. I hate to use him as an example because he is an easy comparison, but Hemingway novels are almost the exact same. The agony and pain of Old Man and the Sea just cannot be reproduced in a film, no different than the beauty of music cannot be reproduced in words. Some works of art are just not meant to be remade in different formats (or at least not under the same title).
If this movie were titled "The Roaring Twenties" instead of "The Great Gatsby" then it would probably be received better than what it is.