The product id type field looks at product id number and assigns GCID / UPC / EAN according to the number of digits.
According to Amazon:
The standard-product-id must have a specific number of characters according to type: GCID (16 alphanumeric characters), UPC (12 digit number), EAN (13 digit number) or GTIN(14 digit number). Please ensure that leading zeros do not get lost when the file is exported from excel to text. This can be accomplished by formatting the numbers in these cells as text, and double checking your .txt file to ensure that no errors have occurred while exporting your file.
Therefore you can create a UPC attribute (for example), attach it to all items and then map it to product id type:
deleteAttribute [amazon product-id-type attribute] mapAttribute [upc attribute] to [amazon product-id-type attribute] setParam localized_name "[amazon product-id-type local name]"
[amazon product-id-type attribute] and [amazon product-id-type local name] are found in the Amazon Template files (they can vary from template to template)