The litre (or liter; US spelling) is a unit of volume and is a non-SI metric unit with the symbol L).
1 litre is equal to the volume in a cube with edges all measuring 10cm. There are 0.22 imperial gallons in a litre. Conversely, 1 imperial gallon is equal to 4.5461 gallons.
1 litre of water weights exactly 1 kilogram.
After the metric system was introduced in France in 1791, it took a couple of years for the entire country to implement it in everyday use. After much backlash, it was decided that the cubic metre was too big for everyday use. By 1795 it was announced that the former 'cadil' (0.001 cubic metres) had been given a new name; 'litre'.
The US gill is a unit of volume in the US customary unit system and uses the symbol gi.
It represents 1/4 of a pint - so in this instance, it is 1/4 of US pint - therefore measures exactly 118.29411825 ml. It's also equal to 8 tablespoons, 24 teaspoons and 5/6 imperial gills.
The gill has been mentioned a number of times in popular literature; including in Treasure Island, Animal Farm and in Dan Simmon's 'The Terror' - and all references are pointing to the use of the gill in terms of alcohol measures or servings.