This guide shows how you can download a private/restricted file from google drive to your app, using a google apps script web app. Expanding on this, it also demonstrates how to download a directory structure from google drive and replicate it on an android device.
Nice, but encoding the bytes to Base64 is not a good idea, it increases the size much. It's better encoding in a hexadecimal format which a little decreases the size. Also compressing the data before sending it using any Deflate/Zip method and decompressing in the app using JS or some tool would be good.