I would expect that email sent through something like gmail is private. Even if there's some possibility of interception under search warrants or stuff like that, I wouldn't expect the email text to be used for training Google AI. I don't use gmail (at least from the client side) anyway, but I think there was some noise about this a while back. Certainly I'd expect a paid LLM server to not train on the prompts. I don't use those servers though, and haven't checked the TOS. I might fool around with some open models sometime but have no interest in ChatGPT.