Thank you lian, but suppose you don't know if a gallon is larger than a liter, or the other way around, so you can't be sure it is "many liters in a gallon" or "many gallons in a liter", what would you ask then?
True, good point. Normally, I know which one's bigger, in this case, I know that gallons are bigger. If you really don't know, you could ask, "how do gallons compare to litres?", or something like that.