Retrieve Google Merchant ID
Connect to your Google Merchant Center account. You will find your Google Merchant ID at the top right of the page.
Retrieve Google Credentials
In your Google Merchant Center, at the top, click on the wheel to access the Settings menu, then click on Content API.
Click on the Authentification tab then click on the plus button to create your Google Merchant Credentials. You will be able to generate and save the a json file with your API credentials.