The data included in the Google product feed by default is already defined for the products in your store through the regular X-Cart product fields that are usually displayed in the storefront for the customers. However, the Google product feed might include the information that you don’t want to show in the storefront not to tire your store's customers with unnecessary details, like google_product_category.
The attributes that you don’t need to show to the customers in your online store but need to submit to Google in the feed can be defined through the global attribute of a hidden field type.
To assign hidden attribute values to your products:
First, create hidden attributes as described here.
If you sell cars and you need to add a "google_product_category" attribute, you should use ‘916’ as a hidden attribute value that corresponds with “916 - Vehicles & Parts > Vehicles > Motor Vehicles > Cars, Trucks & Vans” Google product category according to the Google product taxonomy.
Assign hidden attributes to your products using the Bulk Editing tool.