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 gill is a unit of volume in the imperial unit system and uses the symbol gi.
It represents 1/4 of a pint - so in this instance, it is 1/4 of an imperial pint - therefore measures exactly 142.0653125 ml. It's also equal to 2 imperial cups and 1/32 of an imperial gallon.
It is pronounced with a "J" sound at the beginning - and it actually part of a humorous in the nursery rhyme 'Jack & Jill'.
In the UK it was used throughout pubs as standard measures; for example spirits were served as 1/6 gill as standard. In Scotland it was 1/5 gill and in Ireland 1/4 gill. It's still used in the Republic of Ireland today.