The phrase "gallon water" refers to a specific amount of water, equal to four quarts or 3.785 liters. It is commonly used to measure large quantities of water, such as in bottles or containers. Full definition
The device has a hefty gallon water tank, which allows it to operate for up to 12 hours on a single filling, and it should run quietly. (businessinsider.com)
It heats up in two minutes or less, and each fill of the 3/4 - gallon water reservoir provides 1.5 hours of steam power. (thespruce.com)
Plus, taking a shorter shower can save thousands of gallons a water per year. (longwaitforisabella.com)