^"Gaulish" and "Gallic" are both valid - "Gaullic" OTOH isn't. ¯\_(ツ)_/¯
Funfact: the Irish word gall 'foreigner' (pl. gaill) is derived from Gaul, "who in pre-Viking Gaelic history were the archetypal "foreigners". Dumville says that "what [the Gauls] had done in Gaelic prehistory to gain that status is unknown and was probably unpleasant."" (Caesar recorded some of the Gallic tribes of the Atlantic coast were very accomplished shipbuilders and seafarers and Ireland isn't too far away by sail...)