Is it correct to address an envelope to an Italian man as 'Gentmo'?

Yes, it's acceptable to precede an Italian man's name with 'Gentmo' on the envelope. The letters stand for 'Gentiluomo'. It's a masculine gender noun that means 'gentleman, man of honor, nobleman'. It often is used in conjunction with other titles before the addressee gets to the Italian's actual first and last names. For example, it may be followed by 'Sig.', 'Prof.', 'or 'Dot.' for the respective titles of 'Signore' ['Mister, Sir'], 'Professore' ['Professor'], and 'Dottore' ['Doctor'].
Yes, it's acceptable to precede an Italian man's name with 'Gentmo' on the envelope. The letters stand for 'Gentiluomo'. It's a masculine gender noun that means 'gentleman, man of honor, nobleman'. It often is used in conjunction with other titles before the addressee gets to the Italian's actual first and last names. For example, it may be followed by 'Sig.', 'Prof.', 'or 'Dot.' for the respective titles of 'Signore' ['Mister, Sir'], 'Professore' ['Professor'], and 'Dottore' ['Doctor'].