The Difference between the 'Google Product Category' field and the 'Product Type' field
Basically the Google Product Category is by the Google's category rules and the Product Type is your own category of the items.
The Google Product Category attribute in the Google Shopping extension contains the category of the product only according to Google's product taxonomy rules. Whereas the Product Type attribute contains the category of the product according to your own personal magento taxonomy.
The Google Product Category attribute is only required for all items that belong to the 'Apparel and Accessories', 'Media', and 'Software' categories, and are in feeds which target the US, UK, Germany, France, or Japan. But is it still recommended to send this information if it is not in one of these required areas.
The Product Type field is also strongly recommended for all items if you have a categorization for any of your items but is not required. Please include the full category string. In the feed you can configure the category for Google and use your own category structure like;
<g:google_product_category>270 - Electronics > Communications > </g:google_product_category>
<g:product_type>Electronics > Stands Cell Phones</g:product_type>
If you want to get a list of Google's Taxonomy file rules go to the following link and you can download it from here in the section called 'Downloadable Taxonomy' : http://www.google.com/support/merchants/bin/answer.py?answer=160081