Litres.
Gallons are an Imperial unit of measure.
The metric unit used to measure the mass of a pencil would be grams (g).
You would use the metric unit "liters" to measure the capacity of a hot water tank.
You would typically use the metric unit of centimeters to measure the growth of your hair.
You would use the metric unit "kilograms" to measure the weight of a bag of groceries.
kilograms
The metric unit for liquid volume is liters. Throughout Europe and much of the world gasoline is sold in liters. North America still uses gallons but that is not a metric or SI unit.
You would use the metric unit "kilograms" to measure the weight of a sack of peaches.
You would use the metric unit "kilograms" to measure the mass of a wardrobe.
The metric unit used to measure a picnic cooler would be liters (L) for volume.
Gallons
gallons