Is a gallon of orange juice a discrete or continuous variable?
That's not a right question.
IF (and only if ) orange juice could only be produced in exact gallon units, you could count the units, and it would be a discrete variable, like in the question: "How many gallons of OJ does a familiy buy every month" (provided they can't buy anything but whole gallons).
In all other cases the variable will be continuous , as you can drink or buy any amount of OJ, even half a liter.
Count => discrete
Measure => continuous