1) Go to http://console.developers.google.com/
2) Select a project or create a new one
3) In API & Services (on the left menu), click on Credentials and gen a API Key.
4) You might also want to add an OAuth key for non-public stylesheets
5) In the Dashboard (on the left menu), click on ENABLE APIS AND SERVICES
6) Request access to the Google Sheet API
7) In Unity, open Window > GTSU > Open Config
8) In the Private Tab, set your client ID and Client Secret Code from your OAuth key
9) Request an access code by pressing on "Get Access code", you can then authenticate.
10) Set your API Key in the Public Tab.
For code examples, look it up. :)