You need to start the OAuth dance fi4stfirst, to get the access token.
As per your configurationYou can start it by hitting the Authorize path
that you will need to hit thishave configured in your browser
app, which in your case is http://localhost:8081/authorize
After that you will be redirected to Google sign in page. If the Google oauth credentials are configured correctly the mule app will get the access token and use it for the future requests.