The authors agree that the surname has a Sicilian origin and that one of the main etymological routes points to the Sicilian dialectal term gàgghiu, meaning "pied, dappled or of irregular colours". E. Rossoni defines it as referring to something "motley, painted", possibly in allusion to the colour of the hair, while V. Blunda specifies that the word designated the two-coloured coat of horses and mules, and proposes that the nickname could have been applied both to a person of irregular pigmentation and to the owner of an equine of striking coat.

Rossoni adds a second independent hypothesis: the surname could derive from the medieval name Gaglio, an Italianization of the Germanic personal name Geil, which in Old High German means "energetic, vigorous" or, according to other sources, "lively, cheerful". He also rules out as improbable a connection with the term gaglio (rennet), used in the making of cheeses. The surname is peculiar to southwestern Sicily.