In The American prison experiment, C.J. Ciaramella mentions, “Inmates were given a slice of bread and three gills of water (about one and a half cups) a day.”
That “about” caught my eye, because the gill isn’t a metric measure; it’s from the same customary system as the ounce and cup — and the barrel and hogshead. In fact, the gill is exactly four ounces, or half a cup.
What I never realized from occasionally reading the word in old books though is that it is not pronounced like the “breathing” organ of a fish. The g is soft, pronounced like a j.