Neither that Gemara nor Rashi say that one is not allowed to daven for goyim. There is no such halacha. (If so, where is it in the S’A? However, some such as Sefer Chassidim hold that one should not daven for a non-Jew who is doing evil to the Jewish people, which makes sense.) The Gemara and Rashi only discuss the very specific situation of using the word “Shalom,” which is one of Hashem’s names, to greet goyim.
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