US copyright
Published since 1978: 95 years from publication or 120 years from creation whichever is shorter (anonymous works, pseudonymous works, or works made for hire)
Published 1964–1977: 95 years from publication, 28 if copyright not renewed
Published 1923-1963: 95 years from publication
Copyrights prior to 1923 have expired.
As for the Great Gatsby, since it was published 1925 + copyright renewal, the earliest it would expire would be 2020, possibly 2048 or later if the renewal filed in 1953 stands, even though the work was published before the new renewal laws that took place in 1964.
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