Nigel P: The definition of a 'gill' as 1/4 pint much predates Wikipaedia.
The standard pub measure for spirits (rigidly enforced) used to be 1/6 of a gill (1/24 of a pint)

True for pub measures and also generally in many places, but in Yorkshire where I grew up "a gill" wqas definitely used to mean a "half pint" in non-pub scenarios. We used to get "2 pints and a gill" of milk every day, for instance.