c.1300, from Old French goune "robe, coat, habit, gown," from Late Latin gunna "leather garment, skin, hide," of unknown origin. Used by St. Boniface (8c.) for a fur garment permitted for old or infirm monks. Klein writes it is probably "a word adopted from a language of the Apennine or the Balkan Peninsula." OED points to Byzantine Greek gouna, a word for a coarse garment sometimes made of skins, but also notes "some scholars regard [Late Latin gunna] as of Celtic origin."
In 18c., gown was the common word for what is now usually styled a dress. It was maintained more in the U.S. than in Britain, but was somewhat revived 20c. in fashion senses and in comb. forms (e.g. bridal gown, nightgown). Meaning "flowing robe worn as a badge of office or authority" is from late 14c., on image of the Roman toga. As collective singular for "residents of a university" (1650s) it usually now is opposed to town.