If you want to be able to get your epubs/ebooks from your Google Drive using Koreader, here’s a free way to do it:

1. Put your ebooks into Google Drive (I use FBReader, but you can just use a folder in Google Drive)
2. Sign up for a free 10GB Koofr account
3. Add Google Drive to your Koofr account
4. Create a password to access Koofr via WedDav (it’s at the bottom of https://app.koofr.net/app/admin/preferences/password )
5. Add Koofr as a WEBDAV cloud storage option in Koreader and use the password you created in the previous step
   

Now you can access and download your ebooks from your personal Google Drive storage from within Koreader!