App provides two different kinds of Product ID formats. One is based on Shopify's Google Shopping Feed app and other is SKU itself is submitted as product ID. Please keep in mind that these IDs don't have to be MPN or SKU or GTIN, they just should be unique numbers or alpha-numeric strings for each product or variant submission.
Here both the examples are discussed.  

1. Shopify's format: Shopify_{CountryID}_XXXX_YYYYY, most widely used on Shopify for almost all the different kind of submissions.
2. SKU as Product ID: App provides a feature where you can choose SKU as Product ID attribute that is submitted to Google Shopping. If you prefer to use this option, make sure each and every variant has unique SKU value. If any of your product/variant does not have any SKU, App will not submit that product/variant. If there are two or more product/variant having same SKU, only one product/variant shall be submitted to Google.  

Here is a quick link to install the app: https://apps.shopify.com/google-shopping-feed

For any further query, feel free to connect with us at support@simprosys.com