Is 25 grams equal to 1 ounce?

A gallon = 4 quarts = 8 pints = 16 cups = 128 fluid ounces. So you'd have 25/128, which, converted to decimal, equals 0.1953125. Rounded up to 0.2, you have roughly the same amount as what people refer to as a "fifth". An exact fifth of a gallon is 25.6 fluid ounces, but no one is going to miss the 0.6 ounces, as that is about half of a shot-glass-full. ;-)