The Great Gatsby, the songs of blues singer Ma Rainey, and Franz Kafka's The Trial entered the public domain on January 1st, 2021. Because these works are now free for people to republish without restriction, that means they’re also being uploaded in full to fanfiction site Archive of Our Own—usually with a few changes.
When a piece of fiction enters the public domain, it means it belongs to everyone, and can be copied and sold without the need to seek out a license from the holder of the copyright. You can use it in any way you like, including making adaptations of it, or reproducing it in full. Sherlock Holmes is a character that is in the public domain, though the estate of Sir Arthur Conan Doyle still holds a copyright on stories published after 1932. What this means is that story elements that are present in the earlier Sherlock Holmes stories, which is most of them, are in the public domain, leaving creators free to, say, make a movie about Sherlock Holmes's hypothetical sister Enola, or goof on Moby Dick or to reprint Marx and Engels' Communist Manifesto in full.
So too have enterprising writers now taken to Archive of Our Own, uploading the entire text of The Great Gatsby with usually minor changes. One of them changes the plot to be about characters from the cartoon Steven Universe; another replaces the characters with ones from anime My Hero Academia. Another largely leaves the text unchanged, except for the final line, "And so with our brains small, our meat huge, our pain endless, we tread onwards." My favorite of the half dozen or so works that have appeared on AO3 since the first replaces all instances of the word "Gatsby" with "Gritty."
Although pieces of fiction fall into the public domain naturally after a certain amount of time, determining when has become more complicated over the years. In 1998, the Copyright Term Extension Acts allowed corporations like Disney to extend the copyright on their works to up to 75 years, lest their moneymaker mouse fall into public domain. To be clear, this doesn't have anything to do with creativity or artistry, or allowing creators of a work to take ownership of it. It's basically all about making sure Mickey Mouse doesn't become public domain, and it's depriving us of the creativity on display in adaptations of The Great Gatsby like this one, which is completely unchanged except Tom Buchanon and Jay Gatsby kiss in Chapter 7. For several years, in fact, no works entered the public domain because of this law.
Sherlock Holmes as a character has been successfully reimagined and adapted for our times, recasting the detective as a woman, or placing him in modern times. Allowing people to remix characters that are over a century old seems like it can only be a win for our culture. You could even make an incredibly stupid and bad adaptation on the BBC where a man is killed by his own boomerang—no one can stop you!