Can I say Loshon Hara about another Jew to a Gentile? For example: If Reuven tells Roy that Yehudah is a stupid person. Is Reuven doing an Aveira?
Not sure if just saying someone is stupid is Lashon Hara, it depends on the context and how it's said.
But in general, when slandering to a Gentile, besides the sin of speaking slander about your fellow man, you are also desecrating the name of God by bad-mouthing his children.
"One who violates this and slanders a Jew before gentiles flagrantly rebels against Torat Moshe Rabbeinu (the Torah which Moses taught us), as is discussed in the Shulchan Aruch, Choshen Mishpat 26."