Add Global Trade Identification Numbers (GTINs) to WooCommerce products

functions.php

/**
 * Render the Global Trade Identification Number (GTIN) meta field.
 */
function woocommerce_render_gtin_field() {
   $input   = array(
      'id'          => '_gtin',
      'label'       => sprintf(
         '<abbr title="%1$s">%2$s</abbr>',
         _x( 'Global Trade Identification Number', 'field label', 'my-theme' ),
         _x( 'GTIN', 'abbreviated field label', 'my-theme' )
      ),
      'value'       => get_post_meta( get_the_ID(), '_gtin', true ),
      'desc_tip'    => true,
      'description' => __( 'Enter the Global Trade Identification Number (UPC, EAN, ISBN, etc.)', 'my-theme' ),
   );
?>

   <div id="gtin_attr" class="options_group">
      <?php woocommerce_wp_text_input( $input ); ?>
   </div>

<?php
}

add_action( 'woocommerce_product_options_inventory_product_data', 'woocommerce_render_gtin_field' );

/**
 * Save the product's GTIN number, if provided.
 *
 * @param int $product_id The ID of the product being saved.
 */
function woocommerce_save_gtin_field( $product_id ) {
   if (
      ! isset( $_POST['_gtin'], $_POST['woocommerce_meta_nonce'] )
      || ( defined( 'DOING_AJAX' ) && DOING_AJAX )
      || ! current_user_can( 'edit_products' )
      || ! wp_verify_nonce( $_POST['woocommerce_meta_nonce'], 'woocommerce_save_data' )
   ) {
      return;
   }

   $gtin = sanitize_text_field( $_POST['_gtin'] );

   update_post_meta( $product_id, '_gtin', $gtin );
}

add_action( 'woocommerce_process_product_meta','woocommerce_save_gtin_field' );