I don't recall every hearing any rabbi telling gentiles what they have to believe in. I have listened to many interviews of rabbi Kahane and one of his major talking points for the gentiles is that we simply want to be left alone in our country, don't tell us what to do like we don't tell you.
Tosafot, Tractate Hagigah 13a, states that it is an obligation for the Jews to teach and inform the Gentiles of the Seven Noahide Commandments. Likewise, Moses was also commanded by G-d to compel all nations of the world to accept the seven Noahide precepts that they had been commanded (Rambam, Laws of Kings 8:10).
See also Rambam's Sefer Ha'Mitzvot, Positive Commandment 3, that Jews are commanded to love G-d, and to therefore call out to the people of the world to believe in G-d and recognize Him, quoting Sifri (on Deuteronomy 6:5): "Bring love of Him to all people, like Abraham your father."