You can find "gentes" in cases like this :
Derecho de gentes. From ius gentium, in the Roman Empire the part of the law and the institutions that dealed with the non-citizens. Coming from there, "derecho de gentes" is applied today to international law (that is also called in Spanish "Derecho internacional").
Pobres gentes. The title of a Dostoyevski's novel I haven't read.
To me, "gentes" sounds a bit more "special" than "gente" (that is, I would see it in an Encyclopedia rather than in a conversation with my friends). But as Lore said, they're the same.