Earlier quoted context omitted.
Wait, so you trust them to store the email, and parse it for something like spam detection, but not for some smarter feature? What specifically are you concerned about that is a problem with this and not with them having the email in the first place? Sounds like you shouldn't be using Gmail at all tbh.
You trust snail mail carriers not to read your mail. The digital equivalent of the envelope is encryption - the carrier is allowed to see the outside but not the inside. The function of encryption is lack of comprehension - the carrier still knows the exact encrypted bytes of your mail, but cannot comprehend it. We generally accept that this is already a useful proxy for privacy. Imagine a mail protocol that somehow…
The answer here is privacy laws and auditability.