If you happen to sell products that all fit into the same base category AND your products are NOT required to have the google_product_category attribute, you can quickly apply Google Product Category to every product in your feed.
While NOT REQUIRED in select cases, adding Google Product Category can improve your data quality and provides more information to Google Shopping.
A furniture retailer wishes to classify all products simply as 'Furniture.' Using the 'Search Google Category at the top of the Category Mapping page:
Search for category 'Furniture'
Click 'Fill All'
Finish by clicking 'Save Changes'
Note that you can always choose to extend the category tree to its furthest point as long as it matches the products you wish to apply to.