yes, if you are not practicing other opsec and are emailing with someone who is not using proton.

"Gmail uses TLS (Transport Layer Security)(new window) to encrypt emails by default. That means your messages are secure while in transit, as long as the recipient’s email service supports TLS. But once they arrive, the privacy of your emails depends on what encryption the receiving server uses."

Gmail also holds the encryption keys to your messages. So it can access them and hand over their contents to third parties, like advertisers or governments.

https://proton.me/blog/how-to-send-encrypted-email-gmail