Q. Are Gentiles included in the speaking lashon horah prohibition?

A. Horav Shlomo Miller’s Shlit’a opinion is that Gentiles are not included in the speaking lashon horah prohibitions, since these proscriptions are not part of the seven Noahide Mitzvos. (Sanhedrin 56a). However, when the lashon horah causes damage or harm to others, as it often does, it would be included in the Noahide mitzva of not stealing from and harming others.
Poskim disagree the exact definition and nature of the mitzva of dinim or carrying out justice. It definitely includes a requirement that Gentile societies establish courts and prosecute those who violate the Noahide laws (Tosefta, Avodah Zarah 9:4; Rambam, Hilchos Melachim 9:14). Ramban (Bereishis 34:13) contends that the mitzva of dinim also prohibits injuring or abusing others or damaging their property.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a