This is just impossible. If they're going to be sending your email to gmail then they need to see what's in it. So they will have the data at some point. You have to trust their brown eyes that they don't look at it while it's going through their inbound and outbound servers. But they're selling it as a technical protection, not a trust-based one.

Personally, if you want private Comms, just don't use email. The protocol is just not suitable.