The surname Gallo and its numerous variants constitute a typical case of multiple formation in Italian onomastics, with diverse origins that may be nickname-based, anthroponymic, toponymic and even ethnic. The common base is the Latin term gallus, which means "rooster" (the bird), documented already as a personal name from the 8th century.

According to E. De Felice and F. De Laurentiis, the surname derives in many cases from a nickname, often jocular or descriptive, linked to the behaviour or appearance of the person: someone proud, haughty, of strong character or even of striking dress, in reference to the rooster. In the same line, this term could also be used in a figurative sense in rural contexts to designate a dominant figure within a group. On the other hand, both authors note that Gallo could also have an ethnic value, identifying a person native to Gaul or to France.

In agreement with this, E. Rossoni highlights that these surnames may have different starting points: some derive from the medieval name Gallo, others from the ethnic term, and others from numerous place names present throughout Italy, such as Gallo (Ferrara), Gallo Bolognese (Bologna), Gallo di Grinzane (Cuneo), Gallo di Petriano (Pesaro e Urbino), Gallo di Tagliacozzo (L'Aquila) or Gallo Matese (Caserta), among many others.

The diffusion of the surname could also have been favoured by the cult of Saint Gall, an Irish monk whose devotion spread especially in northern Italy, reinforcing the presence of the personal name from which many of these surnames derive.

From this common base numerous variants arise, the product of suffixation, diminutives and dialectal adaptations, among which are: Galli, Galluzzi, Galluzzo, Galloni, Gallone, Gallarini, Gallucci, Galluccio, Galluccini, among others. In some cases, such as Galluzzi, they may also be linked to specific place names such as il Galluzzo in Tuscany.

As for its diffusion, Gallo is a surname widely spread throughout Italy, with high concentration in Sicily, Campania, Calabria and Puglia, and important nuclei also in Lombardy and Piedmont. Its variants present more specific regional distributions, especially in the centre and north of the country.

In synthesis, Gallo is a surname of multiple origin that may derive from a nickname linked to the animal, from a medieval personal name, from a gentilic or from a place name. Its wide diffusion and the richness of its variants reflect both the Italian linguistic diversity and the superposition of historical and cultural traditions in the formation of surnames.