According to the website, the game was found at a yard sale and was originally planned as a Nintendo game. It came complete with a manual and everything. What you'll play, supposedly, is an emulation of that game. True to form, The Great Gatsby has clear Nintendo roots, with classic 8-bit platforming gameplay, a lovely soundtrack and some completely ridiculous enemies.
Across the game's four levels you'll play as the novel's narrator, Nick Carraway. In typical roaring '20s fashion, you'll pick coins along the way and snagging a martini will net you a health block. Oh, and you'll be throwing your hat at enemies to kill them. Just like in the novel. The story is told through a couple cut scenes and some in-game dialogue. You'll visit Gatsby's mansion, the train, New York (including the sewers) and finally, West Egg Beach, where you'll fend off ghost soldiers (possibly Gatsby's ghost?), crabs and jumping fish. Amazingly, the final line still has as much of an impact as it originally did in the book.
Actually, that end scene works as an homage to two things: to the elusive green light in The Great Gatsby and to the Mario and Castlevania games of the same generation. It's well worth playing through the 10 minutes or so it takes to beat the game to get to the ending sequence. Plus you'll be able to tell everyone that you finished The Great Gatsby today, and they'll high five you and be forced to think about how they never finish anything, let alone a classic novel.
Before you get there, you'll have to master some wonky controls, though, with the arrow keys used for movement, the spacebar used to jump and the tab or 'z' key used to throw your hat. It kind of turns your hand into a pretzel, but it's possible to get used to it.
You can play the game for yourself here.