The gallon is a unit of volume in the imperial unit system with the symbol gal.
1 British imperial gallon is defined as 4.54609 litres or 277.4194 in³.
The term gallon derives directly from the Old Northern French term 'galun' which means 'liquid measure'. The origin of the word prior to this is still unknown. It originated as the base of the system for measuring wine and beer in the UK and has now been adopted across the world and there are similar versions in the US and associated states (although they measure slightly differently).
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.