Simply, do not use GMail as it is a security risk. Use your own or trusted email server, or encrypt sensitive contents. It's what the real bad actors have been doing for decades.
And it is a sure fire way to get your e-mail tagged as spam and rejected/trashed unless you are big corporation, passing your e-mail through a large ISP/vendor (and then you are back to square one) or willing to deal with a ton of hassle, aggravation and costs figuring out why your e-mails are not being delivered, getting lost, bounced, etc.
That is in addition to spam, malware and constant security related stuff (updating the server, hardening it, warding off constant hacking attempts by bots, ...) which are the daily bread and butter for anyone running any kind of server on the internet.
Sadly the time when anyone could run their own server without issues as long as they were a bit technically competent are long gone.
Also if you start encrypting, that's only going to attract even more attention to you because you are standing out like that proverbial guy in a gorilla suit in the middle of party crowd - "Hey look - that guy has something to hide!"
Doesn't mean you shouldn't do these things but it is far from a clear-cut and obvious solution. The tradeoffs are fairly significant there.