Thanks to following code any WooCommerce coupon code can be used to purchase a gift card.

If the customer try to do it, the action will be denied and a message will be showed to him/her "Coupon cannot applied to gift card product"

Add the code in the file functions.php of your theme.

 

if( defined('YITH_YWGC_PREMIUM') ){
    if( !function_exists('yith_wcgc_deny_coupon_on_gift_card') ){
        add_action('woocommerce_applied_coupon','yith_wcgc_deny_coupon_on_gift_card');

        function yith_wcgc_deny_coupon_on_gift_card( $coupon_code ){
            global $woocommerce;
            $the_coupon = new WC_Coupon( $coupon_code );
            $excluded_items = $the_coupon->get_excluded_product_ids();
            $items = $woocommerce->cart->get_cart();
            foreach ( $items as $item ):
                if( has_term('gift-card','product_type',$item['product_id']) == true ):
                    $excluded_items[] = $item['product_id'];
                endif;
            endforeach;
            $the_coupon->set_excluded_product_ids($excluded_items);
            $the_coupon->save();
            wc_add_notice( 'Coupon cannot applied to gift card product', 'error' );

        }
    }

}