Litres.
Gallons are an Imperial unit of measure.
you would use gallons
gallons
metric unit
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.
kilograms
A microgram.
Gallons
gallons
1 US gallon = 3.78541178 liters. Only if you mean pure water, then 1000 litres of water weighs 1 metric tonne. 1000 litres of water are 264.172052357 gallons that weigh 1 metric tonne.
meters
Tonnes