How do I tell Google there's no GTIN, MPN or UAC in my Google Feed?

If you don’t have a manufacturer’s UPC code which meets the GTIN standard you can insert this into your Google feed: :identifier_exists FALSE
See the Directive from Google which states that the g:identifier_exists flag should be false if an item does not have a valid GTIN or UPC.

Please contact Nitrosell Customer Support if you have any questions on the next two steps:

You need to add an attribute in PAM called NOIDExist and set it to ‘TRUE’ for products which DON’t have valid Brand, MPN and GTIN Values. Products which do have valid a MPN or GTIN can be left blank because it defaults to False. Next map PAM_NOIDExist to weblinxcustomtext6 as follows:

In NSc Sync Configuration, under Advanced, select Field Mappings.
Right-click the Item table, Select Add Field, Add Standard Field, specify product_weblinxcustomtext6
For Field Source, specify the following:

PAM_NOIDExist

Then in WSM, Products->Product Feeds, map the custom text field to g:identifier_exists in the Google Shopping config.

Google Validation for GTIN is 10,12 or 13
http://productforums.google.com/forum/#!topic/merchant-center/-DzXwN2MvnI

Google Validation of UPC
12 digits for North America
13 for UK.
10 also in some countries.