Gallons are an Imperial unit of measure.
Capacity can be measured using both the metric system and customary units. In the metric system, capacity is typically measured in liters or milliliters, while in the customary system, it is measured in ounces, cups, pints, quarts, and gallons.
It is a customary unit.
Gallon is an Imperial measurement. It originated in England.
The opposing systems are imperial and metric, not customary. But, meters are metric.
The pound is NOT a metric unit.
It is customary.
It can be more or less. An once (customary) is bigger than a gram (metric) but a pound (customary) is smaller than a kilogram (metric).
Metric.
Metric.
Metric
A "gran" is a German measurement, it is neither customary or metric. A "gram" is a metric unit.
It is a customary unit.