How many gallons are in a cubic gallon?

That question should probably be phrased differently. Gallons are a measure of volume. So are cubic inches and cubic feet. There are 231 cubic inches in one 1 gallon of fluid, and if that fluid is water, then it weighs 8.32 pounds. There is no such thing as a cubic gallon.

Cubic is a term that implies a quantity raised to the power of three. Linear distance measurements -- inches, feet, centimeters, etc. -- can be cubed. Multiplying three terms that use the same units -- length, width, and height, for example -- results in an answer in cubic units, which implies something that takes up space and has a volume. So perhaps you really meant to ask, "How many gallons are in a cubic foot?"