gtin missing

[goaround]

Currently there are just the Sub-properties gtin12, gtin13, gtin14, gtin8 readable not the main property gtin e.g. on Product: https://schema.org/gtin

[jhard]

This can be remedied by adding gtin to the lists in properties.php where gtin8 is available, and documenting it in Interfaces/Product.php as

 * @property-read SchemaTypeList<Text>                       $gtin                      A Global Trade Item Number (GTIN). GTINs identify trade items, including products and services, using numeric identification codes.