Gallon is unit of volume in imperial and U.S. customary systems. After the metric system is adapted, many countries replaced it with liter. In u.s. customary system, there are two types of gallons (fluid and dry) and they are different in sizes, in imperial system there is only one type gallon. It's commonly used unit of volume such as measuring the fuel efficiency as miles per gallon, etc. The abbreviation is "gal".
Fluid ounce is an imperial and U.S. customary volume unit. Fluid ounces are volume units and should not be mistaken with ounces which is mass (weight) unit. The abbreviation is "fl oz".