gtin property
//en.wikipedia.org/wiki/Global_Trade_Item_Number) for more details.
Left-padding of the gtin values is not required or encouraged. The
Schemagtin property generalizes the earlier Schemagtin8,
Schemagtin12, Schemagtin13, and Schemagtin14 properties.
Note also that this is a definition for how to include GTINs in Schema.org data, and not a definition of GTINs in general - see the GS1 documentation for authoritative details.
Implementation
String? gtin;