The authors agree that this surname originates in the Lombard term hattu or hatto, which meant "combatant". F. Miranda also notes that in the language of the Goths, hadu had the meaning of "combat", and that both terms are closely linked. The forms Hatto and Atto circulated among the Lombards between the 6th and 12th centuries —Miranda mentions that an Attone was a Lombard duke—, and between the 11th and 12th centuries the transformation into the surname Gatto took place.

E. Rossoni documents the surname from the 10th century: it appears in the Codice Diplomatico Padovano of 1080 and in the Codice Diplomatico Veronese of 1291, and also records the medieval form Gactus, with examples in Viterbo in a testament of 1306 and in Pisa in the second half of the 13th century. G. P. Holmes, on the other hand, proposes an alternative etymology from Late Latin with the sense of "cat".

The surname is widely spread throughout Italy, both in the north —Piedmont, Liguria, Lombardy and Veneto— and in the south, including Lazio, Campania, Calabria, Sicily and the Salentine area, that is, the southernmost zone of the region of Puglia. Miranda adds that in Sicily there is also the compound form Gatto Rotondo, present in zones of the provinces of Catania, Syracuse and Enna.