Updating GTIN via CSV file

I have a woocommerce site with over 4,500 products. I have created the custom GTIN field with the plugin and now I am trying to populate the GTIN field by using a CSV file however there doesnt seem to be the option within the woocommerce import tool to map my CSV data (the barcodes) to the GTIN field.

There is the option in the woocommerce import tool to map the barcodes to a custom field but I need to know what the GTIN custom field that I have created will be called so that the woocommerce import tool will recognise it. I have tried mapping the barcodes to “GTIN” and “gtin” but neither worked.