What was F. Scott Fitzgerald's main purpose for writing The Great Gatsby?

According to the foreword to the novel written by Charles Scribner III, Fitzgerald wanted to write a book that was "consciously artistic" and "beautiful and simple and intricately patterned."
Having had commercial success with This Side of Paradise, Fitzgerald hoped, in other words, to write a literary masterpiece. Fitzgerald started to write this novel as a satire called Trimalchio, based on the Roman satire the Satyricon, but the novel transcended that form. Gatsby was modeled on Trimalchio, a nouveau riche former slave who gave lavish parties, but Gatsby transformed in Fitzgerald's hands into a figure of tragic romance.
Fitzgerald also wanted to explore the American Dream, as the novel does through Gatsby's desire to start anew and wipe away the past, which is part of the larger American dream of coming to a new continent and creating a new and improved society. In Gatsby's case, the dream means regaining Daisy and acting as if the years they were apart never happened.
Fitzgerald succeeded in writing a novel that was lyrical, short, and "intricately patterned"—there is an enormous attention to detail in this work. Though not a success on its first release, the novel is now solidly in the canon of great American literature.
https://bigthink.com/purpose-inc/f-scott-fitzgerald-on-writing-the-great-gatsby

There are many purposes in the novel. We look at the disillusionment of the materialistic world, how excessive it is and how corrupted wealth is.

Despite its slim size, The Great Gatsby encompasses a diverse array of important themes, commenting on everything from the state of capitalism to gender relations. As such, it's very difficult to pinpoint a major purpose. That said, if there is a major purpose in the novel, it is most likely Fitzgerald's critique of the classical American Dream.
Simply put, the American Dream is the widespread notion that any American citizen can achieve happiness and fulfillment by simply working as hard as possible. Fitzgerald deconstructs this idea by showing that, though Gatsby works hard and acquires a vast store of riches, he does not ever achieve true happiness or fulfillment. For Gatsby, true happiness involves earning the lasting love of Daisy. However, though Daisy loves him in her own way, she is not able to love him as fully as he would prefer, and in the end Daisy abandons Gatsby. Thus, no matter how hard Gatsby works to gain material wealth, he ultimately dies alone, and so much of the novel's major purpose is to ultimately critique the mythology of the American Dream.