A tonne (pronounced 'metric ton') has dimensions of mass. A cubit is a unit of length, so a cubit metric tonne has dimensions mass x length. The two are incomparable.
1 US gallon = 3.78541178 liters.
Only if you mean pure water, then 1000 litres of water weigh 1 metric tonne.
1000 litres of water are 264.172052357 gallons that weigh 1 metric tonne.