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.
Gmail's native Offline mode works by syncing your account as soon as an internet connection becomes available. You'll be able to write emails, archive or delete stuff offline and know that they'll get sorted out later -- you can also choose whether to download and keep offline data on your computer even if you log out. You can now also take Smart Compose for a spin, if that's what you're looking forward to. Unlike Offline mode, though, you'd have to enable access to experimental features to start getting phrase and sentence suggestions based on previous emails.