What proof is there that "The Great Gatsby" took place in the 1920's?
To me, there is proof that the book takes place in the 1920s because it clearly takes place after 1919 and it clearly takes place before 1929. Here is how I know these things.
The book cannot take place during the Great Depression. There would surely be a mention of the stock market crash or something like that. Besides, it was published in 1925.
The book has to take place after 1919. We know that Gatsby was in the army for WWI, which ended in 1918. We also know that Meyer Wolfsheim was involved in rigging the 1919 World Series.
Therefore, the book has to happen in the 1920s.
If you want more direct proof, we also see at the start of Chapter 4 that the parties that Nick is describing happened in 1922.