Google has released standalone apps for Google Docs and Sheets, with Slides to follow soon.
The standalone apps are available now for Android and iOS, with Google's official blog declaring:
The apps also come with offline support built in, so you can easily view, edit and create files without an Internet connection. Now, if you have a brilliant idea for a best-selling novel while traipsing through the Amazonian rainforest (or you know, something more probable, like during flight takeoff)…no problem. You can jot down your idea in the Docs app on your phone, even when you’re offline.
You can get the apps on Google Play [Docs] [Sheets] and in the App Store [Docs] [Sheets] <~Click links to open in relevant app store. The apps are free.