Gmail has SSL to Google's e-mail server. If you e.g. send a mail to someone using Outlook, it's sent in clear text. And at both ends it's scanned to deliver everyone involved targeted advertising.
"Reasonable expectation of privacy" doesn't have an implicit "only as against the government." If you're knowingly exposing the contents to Google and Microsoft to scan, you can't claim to have a "reasonable expectation of privacy" in the contents.