Actually The Great Gatsby was not really appreciated by critics or the commercial market until after Fitzgerald's death in 1940. During WW2 the book was distributed to soldiers and they liked it. By 1944 critics recognized the revival in interest wasn't a fad, and scholars started to re-examine it. After that it became both a critical and commercial success, and is still both today -- but Fitzgerald never knew about it.

I agree that it's a fantastic novel. But I would say it's only been recognized as such for about 80 years, maybe less.

I wonder why it appealed so much to WW2 soldiers, in particular. It may have had something to do with the waning power of the Eastern elites portrayed in the book. After all, the soldiers were picked from all walks of life. And then when they returned to civilian life they weren't content to go back to their previous roles. They formed the great middle class of the 1950s and 60s, and through the G.I. bill they filled colleges that formerly had been reserved for the rich and privileged.

Now we may have come full circle, with a much greater divide between the rich and the rest of us. But that would make the book more relevant than ever.