Outside of chemistry, liquids are typically measured in litres (AmE: liters), a metric unit of capacity equal to the volume of one kilogram of water at 4°C (39°F) and at standard atmospheric pressure of 760 millimeters of mercury.
"Gallons" are not a universal metric:
For the US, 1 Liter of milk = 0.2641720524 (US) Gallons.
For the UK, 1 Liter of milk = 0.2199692483 (UK) Gallons.
Because this has been and still is confusing, the standard unit of liter was introduced as a universal alternative (no flamebait intended).
"Gallons" are not a universal metric:
For the US, 1 Liter of milk = 0.2641720524 (US) Gallons.
For the UK, 1 Liter of milk = 0.2199692483 (UK) Gallons.
Because this has been and still is confusing, the standard unit of liter was introduced as a universal alternative (no flamebait intended).