Nick Carraway rents a house on Long Island for eighty dollars a month, next door to the mansion where Jay Gatsby throws enormous parties and stands alone on the marble steps watching. Gatsby says he is from the Middle West and names San Francisco; the guests say he is a bootlegger. What he wants is Daisy Buchanan, last seen five years ago and married now to Tom — rich, an old football man from New Haven, casually cruel — across the bay. Tom keeps an apartment in the city for Myrtle Wilson, who lives over her husband's garage in the ash heaps. One sitting does it, and most of that is parties and small cruelties; then the hottest day of the summer, a hotel room, and the drive home.
The Great Gatsby is 48,557 words. At an average adult reading pace of 300 words a minute that is about 2 hr 42 min, or roughly 2 hr 1 min if you read quickly and 4 hr 3 min at a more deliberate 200 words a minute.
That count is measured from the complete public-domain text rather than estimated from a page count, so you can compare all the books side by side.