Old emails still open even after being logged out of gmail

My tablet was stolen, so I've logged out of my google account on all devices and changed my password. But when I test it on my other tablet, my emails still open even though it's not logged in. The tablet tells me that I need to sign in, and I can't send emails, but all the emails that were already in the app are readable. How do I fix it? That means whoever stole my tablet can read all my old emails?