WooCommerce - Add GTIN/EAN meta field for product and product variations

functions.php

/** 
 * Adding Custom GTIN Meta Field
 * Save meta data to DB
 */
// add GTIN input field
add_action('woocommerce_product_options_inventory_product_data','woocom_simple_product_gtin_field', 10, 1 );
function woocom_simple_product_gtin_field(){
   global $woocommerce, $post;
   $product = new WC_Product(get_the_ID());
   echo '<div id="gtin_attr" class="options_group">';
   //add GTIN field for simple product
   woocommerce_wp_text_input( 
		array(	
			'id' 			=> '_gtin',
			'label'       	=> __( 'GTIN', 'textdomain' ), 
			'placeholder' 	=> '01234567891231',
			'desc_tip'		=> 'true',
			'description' 	=> __( 'Enter the Global Trade Item Number (UPC,EAN,ISBN)', 'textdomain' )
		)
	);
   echo '</div>';
}
// save simple product GTIN
add_action('woocommerce_process_product_meta','woocom_simple_product_gtin_save');
function woocom_simple_product_gtin_save($post_id){
   $gtin_post = $_POST['_gtin'];
   // save the gtin
   if(isset($gtin_post)){
      update_post_meta($post_id,'_gtin', esc_attr($gtin_post));
   }
   // remove if GTIN meta is empty
   $gtin_data = get_post_meta($post_id,'_gtin', true);
   if (empty($gtin_data)){
      delete_post_meta($post_id,'_gtin', '');
   }
}



// Add Variation GTIN Meta Field
add_action( 'woocommerce_product_after_variable_attributes', 'variation_settings_fields', 10, 3 );
function variation_settings_fields( $loop, $variation_data, $variation ) {
	// Text Field
	woocommerce_wp_text_input( 
		array( 
			'id'          => '_gtin[' . $variation->ID . ']', 
			'label'       	=> __( 'GTIN', 'textdomain' ), 
			'placeholder' 	=> '01234567891231',
			'desc_tip'    => 'true',
			'description' 	=> __( 'Enter the Global Trade Item Number (UPC,EAN,ISBN)', 'textdomain' ),
			'value'       => get_post_meta( $variation->ID, '_gtin', true )
		)
	);	
}
// Save Variation GTIN Meta Field Settings
add_action( 'woocommerce_save_product_variation', 'save_variation_settings_fields', 10, 2 );
function save_variation_settings_fields( $post_id ) {
	
	$gtin_post = $_POST['_gtin'][ $post_id ];
	// save the gtin
	if(isset($gtin_post)){
	  update_post_meta($post_id,'_gtin', esc_attr($gtin_post));
	}
	// remove if GTIN meta is empty
	$gtin_data = get_post_meta($post_id,'_gtin', true);
	if (empty($gtin_data)){
	  delete_post_meta($post_id,'_gtin', '');
	}
	
}