Asturian gatu, Portuguese gato, Galician gato, Catalan gat, French chat, Italian gatto, Aromanian cãtushe, Romanian cătușă, Sardinian batu
Old Irish catt "cat," Welsh cath 'id.,' Cornish kath 'id.,' all borrowed from Latin cattus
Proto-Germanic *kattu- "cat," which may be borrowed from Latin cattus or borrowed from another language. Old Norse kǫttr "cat," Old High German kazzo "cat," Old Saxon katto 'id.,' Old Frisian katte 'id.,' Old English catt (English cat)
Russian kot "tomcat," Lithuanian katė̃ "cat," borrowed from either Latin cattus or Proto-Germanic *kattu-
Ancient Greek κάττα (kátta) "cat," Modern Greek γάτα (gáta) 'id.'
Old Armenian katu "cat," borrowed from an unknown language
Basque katu "cat," (dialects) gat(h)u 'id.,' borrowed from Late Latin cattus
Arabic qiṭṭ "cat," Syriac qaṭṭu 'id.,' possibly borrowed from another language
The sound shift of /k/ to /g/ did not affect c-initial words in Latin, so we are left with the question of whence the g- in gato? Considering cognates in Catalan gat and Italian gatto, we could reconstruct a Vulgar Latin variant *gattus but this looks unmotivated. To posit that the word was borrowed from another language (Basque?) would be at odds with the historical record.