The Greek and Latin names Galatia (first attested by Timaeus of Tauromenium in the 4th century BC) and Gallia are ultimately derived from a Celtic ethnic term or clan Gal(a)-to-. The Galli of Gallia Celtica were reported to refer to themselves as Celtae by Caesar. Hellenistic folk etymology connected the name of the Galatians (Γαλάται, Galátai) to the supposedly "milk-white" skin (γάλα, gála "milk") of the Gauls. Modern researchers say it is related to Welsh gallu, Cornish: galloes, "capacity, power", thus meaning "powerful people".
The English Gaul is from French Gaule and is unrelated to Latin Gallia, despite superficial similarity. The name Gaul is derived from the Old Frankish *Walholant (via a Latinized form *Walula) literally "Land of the Foreigners/Romans", in which *Walho- is reflex of Proto-Germanic *walhaz, "foreigner, Romanized person", an exonym applied by Germanic speakers to Celts and Latin-speaking people indiscriminately, making it cognate with the names Wales and Wallachia. The Germanic w- is regularly rendered as gu- / g- in French (cf. guerre "war", garder "ward"), and the historic diphthong au is the regular outcome of al before a following consonant (cf. cheval ~ chevaux). French Gaule or Gaulle cannot be derived from Latin Gallia, since g would become j before a (cf. gamba > jambe), and the diphthong au would be unexplained; the regular outcome of Latin Gallia is Jaille in French, which is found in several western place names, such as La Jaille-Yvon and Saint-Mars-la-Jaille. Proto-Germanic *walha is derived ultimately from the name of the Volcae.
Also unrelated, in spite of superficial similarity, is the name Gael. The Irish word gall did originally mean "a Gaul", i.e. an inhabitant of Gaul, but its meaning was later widened to "foreigner", to describe the Vikings, and later still the Normans. The dichotomic words gael and gall are sometimes used together for contrast, for instance in the 12th-century book Cogad Gáedel re Gallaib.
As adjectives, English has the two variants: Gaulish and Gallic. The two adjectives are used synonymously, as "pertaining to Gaul or the Gauls", although the Celtic language or languages spoken in Gaul is predominantly known as Gaulish.