A US gallon is only 3.785 litres. An imperial gallon is 4.54litres.
A 5 litre fill is approx 1/3rd more than a US gallon fill.
1/3rd more powder is therefore 1 and 1/3 scoops
So, for a 5 litre fill, you need one and a third scoops of powder.
These figures are approximate for working practicality.
Safe and happy measuring
Ken
PS If my arithmatic is wrong, please correct me