The authors agree that this surname derives from the Latin personal name Aegidius, whose popular Italian form is Gilio —with the Venetan variant Zilio— and whose semi-learned outcome is Egidio. E. De Felice notes that Gilio was already frequent between the 12th and 13th centuries, from northern Italy down to Tuscany, and that its spread was influenced in part by the French name Gilles and by paretymological crossing with the name Giglio.

E. Rossoni also mentions the medieval Latin form Gillius as a possible intermediary. According to Rossoni, the surname shows specific areas of presence in Emilia-Romagna —between the Bologna, Modena and Reggio Emilia areas—, in Trentino-Alto Adige and in the Turin area.

F. De Laurentiis stresses that the surname spread mainly in northern Italy, and underlines that the semi-learned form Egidio is more usual in the centre and south of the Italian peninsula. He adds that confusion with the name Giglio helped to consolidate the various spelling variants of the surname, among them Gillius, Gillio, Gilius, Gilio, Gilli and Gili.