Answer: A "wine gallon" is a measurement of any liquid's total volume. 1 "wine gallon" is equal to 128 fl oz or 3.78 liters.
A "proof gallon" is a measurement of only the ethyl alcohol contained in a given volume of liquid. 1 proof gallon is equal to 1 wine gallon of liquid containing 50% ethanol.
See below for examples to demonstrate the point.
Mr tincture says:
Because a proof gallon is dependent on the amount of ethyl alcohol present, 1 proof gallon can be represented by many different final volumes of liquid. For example:
Some more proof gallon examples: