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 peck is a unit of volume in the US customary unit system. It uses the symbol pk.
1 US peck is equal to 1/4 of a US bushel. This is also equal to 2 US dry gallons, 8 US dry quarts or 16 US dry pints. In SI / metric, this equates to 8.809768 L.
Although the imperial peck is almost obsolete, it survives in some places in the USA - where some products (namely apples) are still sold by the peck.
It appears in the old English nursery rhyme; 'Peter Piper' where he 'picked a peck of pickled peppers'.