Google collects any content you create, and can read your emails.
Google holds on to any data you have deleted from their service.
Google can store your data if you visit unrelated sites that are using Google services.
Key Points in ToS:
"We also collect the content you create, upload, or receive from others when using our services. This includes things like email you write and receive, photos and videos you save, docs and spreadsheets you create, and comments you make on YouTube videos."
"If you remove from our services any content that’s covered by this license, then our systems will stop making that content publicly available in a reasonable amount of time. There are two exceptions: If you already shared your content with others before removing it. ... [or] if you make your content available through other companies’ services..."
"The activity information we collect may include: ... Activity on third-party sites and apps that use our services."