To add a Google Shopping instance and authorize it please go through the following steps:
1. Go to Settings > Plugins > Google Shopping and turn on the toggle:
2. When the instance has been created, enter your Merchant Center ID and values for "client_email" and "private_key" from the downloaded content-api-key.json - read more in:
How To: Setup Google Shopping Actions. Once filled in, hit Authorize.
4. When Google is authorized, the following section will populate allowing custom fields to be mapped, product import, etc: