I also am having this issue. Is it perhaps a permissions issue with Google? I've tried everything. I can access Drive with the mobile Outlook app.
So I've been having the same issue and found this link which I'm pretty sure explains what the problem is:
http://www.infoworld.com/article/2845623/cloud-storage/cloud-storage-in-ios-81-doesnt-work-as-you-expect.html
It looks like Google have not yet implemented the required xcode APIs to allow Microsoft to save files back into Drive. This is possibly why Drive only works in Outlook - Outlook just needs read only access so all works fine.
I have confirmed that this issue is not limited to Microsoft products - Google Drive is also not accessible via apple's own iWork apps (Pages, etc). I installed box as referenced in the artcile above and all works fine - I can open and save files from box
exactly as expected in all the office apps.
While it's pretty poor form for Microsoft to have advertised this feature when it doesn't yet work as expected in the real world it looks like this is not actuallly their fault or within their power to fix - we need Google to update the Drive app to support
the necessary APIs.
Time to hit the google product forums!!