A little more than 4.5 liters. Except a certain nearby country uses a little less than 3.8 liters as their gallon, so we get all sorts of "smaller than a gallon" packages labeled as a gallon, leading to people getting ripped off if they don't realize they're being sold something less than a gallon as if it were a full gallon.

In other words, the gallon is the original shrinkflation unit of measure.