So having debugging turned on shows nothing at all? Does it 'hang' forever and ever?
How about things like checking true server logs ... assuming apache ... apache error logs ... which, depending upon how one is hosted and config of PHP might be error_log in directories of moodle code.
https://docs.moodle.org/36/en/OAuth_2_services
App verification
NOTE: If you are just using this for document conversion you will not need to verify (as it only uses one account). You will, however, need to verify your ownership of your domain (see, [1])
The Google Drive repository and Google Drive converter also require the app to be verified by completing the OAuth Developer Verification Form. The list of scopes that Moodle requires are:
openid profile email https://www.googleapis.com/auth/drive
Where one gets the Oauth credentials ... in IAM ... there are 3 tabs. Did you complete 'bolded' above as well as the other tab?
'spirit of sharing', Ken