Insert GTIN13 data using Existing Attribute

Hello,

Thank you for contacting the support.

You can modify and add the below filter to the website to add GTIN to the products:

add_filter( 'rank_math/snippet/rich_snippet_product_entity', function( $entity ) {
    if( false !== is_product() ){
	global $product;
        $gtin = $product->get_attribute( 'attribute-1' );  // Replace 'attribute-1' with attribute name
        $entity['gtin8'] = $gtin;  // Replace 'gtin8' with the required GTIN type.
    }    
    return $entity;
});

In the above code, firstly you need to replace replace attribute-1 with the actual attribute name (the slug), and secondly, you can replace gtin8 with the required GTIN type.

Here’s how you can add the filter:
https://rankmath.com/kb/wordpress-hooks-actions-filters/

Hope this helps.

Thank you.