FYI I edited my post and thoughts after learning more info from the source.
But with that said, their TOS for all products are publicly available.
https://www.google.com/policies/terms/
“Our automated systems analyze your content (including emails) to provide you personally relevant product features, such as customized search results, tailored advertising, and spam and malware detection. This analysis occurs as the content is sent, received, and when it is stored.”