I have been trying to use google signin, which is now required, with google spreadsheet api. There are 2 google examples.
I did manage to get the API's to work. There was another Google video that helped. "Sign in series #3: Advanced Topics in Google Sign-in."
After a quite a bit of searching I now have my app adding data to a Spreadsheet! Yeah!
A few things about that.
service.fetchObject(byInserting: .... It does a POST call