The Gill is a unit of measure equivalent to one-quarter of a pint–5 imperial fluid ounces for an imperial gill or 4 U.S. fluid ounces for a U.S. gill.
I remember seeing this unit for a variety of brandies and liqueurs on the menu of The Inn of the Welcome Wench when I played AD&D module T1: The Village of Hommlet.
I don't think I ever had a character order a gill of anything in Hommlet but I learned plenty of new words when I was a kid just starting out playing D&D. It was one of the leading factors in improving my vocabulary and prompted me to read more and more fantasy and science fiction.
The gill is probably appropriate in a fantasy game due to D&D tradition but feel free to shake it up.