I wanted to add a note on what worked for me:
For my setup, flarum is hosted at a URL like https://mydomain.example.com/forum, so in the Google credentials settings, I have the "Authorized javascript origin" set to https://mydomain.example.com (for this post, I'm not using my real domain), and the "Authorized redirect URI" I have set to https://mydomain.example.com/forum/auth/google .
I also jumped through all the Google hoops they tell you to do when adding a client OAuth ID:
- Enable an API (in particular the Google+ API)
- Setup the OAuth Consent screen
- Verify your ownership of the domain
Finally, it is critical to remove the whitespace at the beginning and end of the Client ID and Client Secret when pasting into the extension configuration fields. I had thought I did this correctly, removing spaces from the end, only to realize that when I had copied these, there were also spaces at the beginning!
