The Italian word for 'gentleman' is simply the compound word gentiluomo [djen-teel-woh-moh], with gentile meaning 'gentle' and uomo being Italian for 'man'.
Another compound word that can be used to mean 'gentleman' is galantuomo, but this word enjoys less usage and is mainly employed as part of the phrase un fior di galantuomo meaning 'a perfect gentleman'.
Of course, yet another popular word used to refer to a 'gentleman' is signore. This word, originally meaning 'lord', can also be used in the same way as are the titles 'Sir' or 'Mister' are in English. The salutational phrase signore e signori is the equivalent of the English 'ladies and gentlemen' that you might hear at the beginning of a speech or public address.
One thing to look out for when travelling to Italy is that public toilet facilities will be labelled uomo and donna, simply meaning 'man' and 'woman', rather than the English preference for the use of 'gentlemen' and 'ladies'...
