Depends how deep
There are no gallons of water in a square foot. Gallons is a unit of volume, and square feet is a unit of area, not volume. If you are asking about cubic feet, however, the answer is there are about 7.5 gallons in a cubic foot.
The average swimming pool will hold about 16,000 gallons of water. This is true for a 2700 cubic foot pool.
A third dimension is required, but you can figure about 900 gallons for every foot in the third dimension.
3.35 feet 18 x 18 x 3.35 x 5.9 = 6403 gallons
A 12-ft round pool with depth of 2 ft has a capacity of 1,692.2 gallons of water.
The area of the pool would be 1440 cubic feet. There are 7.4805 gallons per cubic foot. Doing the math, there would be 10771.92 gallons of water in your 15 X 24 X 4 foot swimming pool.
7,480.52 gallons for every inch of depth
770/500 = 1.54 gallons per square foot