It's a bit hidden on this docs page but under API Authentication it says
The "auth verification endpoint" is used to test whether or not the user is currently authenticated. Retool will make a GET request to the URL and if the response is not a 20x, it will pop a modal open and ask the user to authenticate against the API.
That is likely what is happening here because I don't believe the base URL is an endpoint with a valid response. I am not sure what the google drive endpoint is but I used https://www.googleapis.com/calendar/v3/colors for the google calendar API to test it out.