Q. Can one speak lashon horah on a Gentile? If not, can you speak on a Gentile if he also speaks lashon horah on others?

A. Although the Chofetz Chaim in his Seforim does not mention that the prohibitions of lashon harah apply when speaking about Gentiles, the Medresh Devarim Rabba (6: 8-9) quotes Rabi Yochanan saying that if you become accustomed to speak on one that is not from your nation, eventually you will speak wrong on your people too. (See similar in Medresh Tanchuma ”“ Pekudei 7).
Radal in his elucidation (ibid.) maintains that lashon harah on Gentiles is therefore prohibited.
Chut Hashani (P, 357) asserts that if a Gentile who keeps the Seven Noahide mitzvos is involved, it is forbidden. but the prohibition is Rabbinical.
However, Haemek Shailah (28) explains that the Midrash may be referring to a Ger. While Zemanei Reuben (p.252) intents to differentiate between the different prohibitions involved in gossip and rumor-mongering.
The Rosh in Orchois Chaim (Day 5: 83) writes; Do not express wrong and lashon horah on any creature.
Horav Shlomo Miller’s Shlit’a opinion is that as mentioned above it is prohibited since it is the cause of bad midos and grievous character traits.

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