You might have access to Google's native Offline mode very, very soon if it hasn't landed on your account yet. The feature, which debuted as part of Gmail's redesign at Google I/O, has started rolling out to users. If you're using a work or school account, your Google admin will have to enable it in G Suite first. But if you have an ordinary account, you're bound to get it in the near future: to activate it, go to Settings, find the Offline tab and tick "Enable offline mail." Take note that you'll only see the option in the new Gmail (and not in the old interface) and if the browser you're using is Chrome 61 or later.