Yes, a gill is 'officially' a quarter of a (UK) pint, i.e. 5 fl oz.

But in some parts of the country it is used colloquially to refer to a half pint. I suppose it's a bit like the proverbial 'swift half' which usually implies a slightly larger quantity :-)

A 'nip' does often mean a small quantity or 'tot' as Bert says, but it was also used by brewers to refer to a small bottle of ale (usually a strong one such as Barley Wine or Russian Stout) which was sold in 1/3 pint bottles. Maybe some brewers still use this measure?

Something many people don't know is that the 1/3 pint measure (once common) can still be served legally in pubs, but I don't personally know of any that do this. A recent CAMRA beer festival had 1/3 pint glasses. I hope this idea spreads, because it seems very sensible when attending a beer festival, as you can then try more different ales before you assume the horizontal position :-)