Not sure if you have already solved this one since its been some time... but I just ran into the same error and for anyone else that comes across this, here is what my issue was and the solution...
When you run the command in the instructions (https://github.com/splunk/splunk-add-on-google-drive/blob/master/README.md) that takes the client id/secret key and then has you enter the authorization code, what it does is create a local credentials file named splunk_input_name_google_drive_creds which the add-on expects to be within $SPLUNK_HOME/etc/apps/splunk-add-on-google-drive/bin/ ...in my case when I ran this command the file was created in $SPLUNK_HOME so I needed to move it to the proper bin directory. Upon doing so, this fixed the authentication issue but then the Google Reports API returned a 403 error because in the documentation it is missing the part that you must choose the "Admin SDK" from the "Google Apps APIs" after creating your project. If this API is not enabled for your project, then you receive the 403 error.
All of this troubleshooting is made much easier if you enable DEBUG logging or logging to a separate file (not splunkd.log). Check inside $SPLUNK_HOME/etc/apps/splunk-add-on-google-drive/bin/googledrive.py for more information.
Hope this helps.
