The product ID does not appear in the generated feed.

By default, this field contains the product or combination reference code.
If products and combinations in your store don't have reference codes, match this field to a different variable, e.g. the product id.
To do this:
1. clone the configuration you want to modify, e.g. "Google Merchant Center":


2. then go into "export data":


3. click on editing the g:id field:


4. and match this field to the product id: