It's nothing like that.
It's a love story, and a tragedy. It's a short moment in time that has left a great impact on the main character and narrator. It unfolded because of an accident and a mistaken identity. And Gatsby himself is a young man.
The Penguin version that I read had about 50 pages of analysis by way of introduction. I think it was somewhat over-analysed as a study on American class and society (old money v new money/east v west).
I think it was a doomed love story. Gatsby wanted to amass wealth to try to prove his worth to the wealthy woman that he loved. He didn't think he would have a chance with her unless he was rich. He probably didn't, because she was a materialistic snob. In any event, when she discovered that his money was from bootlegging, it wasn't good enough for her then either. In another way, it was a doomed love because Gatsby only loved her whilst she was unobtainable - he yearned from a distance. When she was sitting in his house with him, or at his party, she wasn't what he imagined her to be.
Nick Carraway, the narrator, is very disillusioned by the end of the story, even tainting his story-telling on the way through. He is sick and tired of the snobbery, the materialism, the shallow and the false. He is disgusted by a lot of what he sees. He has become a cynic, and I think he's given up on believing in the good of people.