You've to integrate Google Docs and Google Drive API to enable the API. Please follow the steps below to integrate:

Select your project (1) and click on Enable APIs and Services (2).

Search for Google Drive API and Google Docs API and enable them.

Now go to OAuth consent screen tab.

In Publishing status, there has two option. Production and testing.
Production: Your site should be HTTPS for this mode.
Testing: You can use any security protocol (HTTPS/HTTP) in testing mode. But you'll get an another page on authentication for this, which is mentioned in the last of this steps.

Click on EDIT APP button and fill up with your desired information.

Click Save and continue button on Scopes and Test users step. And click Back to dashboard from Summary tab.
Click on the Credentials tab now from left panel. Click on Create credentials button and select OAuth client ID.

Select Web application for Application type. In 'Authorized redirect URIs', you should add this url https://yourdomain.com/index.php/google_docs_integration_settings/save_access_token. You'll find this in your integration setting.

After saving, you'll get the Client ID and Client Secret. Add them to the settings and click on Save & authorize.

If you've selected Testing mode in OAuth consent screen, you'll get this option. Select Continue from here.

On retrieving from API access, please make sure that, status of Google Docs Integration is showing Authorized.

That's all! You can now create and manage Google Documents inside the CRM.