Php sprintf: combine 2 variables values (string) in %s

Your question is a little bit unclear
You should specify first that you want to edit the loop/add-to-cart.php WooCommerce template that displays add-to-cart button on shop and archives WooCommerce pages.

As it set a GET Url in the href <a> tag like for example:

http://www.myshop.com/shop/?add-to-cart=208 

To make it work you will need to add something like:

http://www.myshop.com/shop/?add-to-cart=208&gclid=601

This will work adding & + gclid= + the value of gclid (here for example 601)…

So the code below will work perfectly (see it in action on this test server):

<?php/** * Loop Add to Cart * * This template can be overridden by copying it to yourtheme/woocommerce/loop/add-to-cart.php. * * HOWEVER, on occasion WooCommerce will need to update template files and you * (the theme developer) will need to copy the new files to your theme to * maintain compatibility. We try to do this as little as possible, but it does * happen. When this occurs the version of the template file will be bumped and * the readme will list any important changes. * * @see         https://docs.woocommerce.com/document/template-structure/ * @author      WooThemes * @package     WooCommerce/Templates * @version     2.5.0 */if ( ! defined( 'ABSPATH' ) ) {    exit;}global $product;// Just for testing (replace after with $gclid=$_GET['gclid'];)$gclid = '&gclid=120';// $gclid=$_GET['gclid'];echo apply_filters( 'woocommerce_loop_add_to_cart_link',    sprintf( '<a rel="nofollow" href="%s" data-quantity="%s" data-product_id="%s" data-product_sku="%s" class="%s">%s</a>',        esc_url( $product->add_to_cart_url().$gclid ),        esc_attr( isset( $quantity ) ? $quantity : 1 ),        esc_attr( $product->id ),        esc_attr( $product->get_sku() ),        esc_attr( isset( $class ) ? $class : 'button' ),        esc_html( $product->add_to_cart_text() )    ),$product );// Checking that you get 'gclid' value (just for test)// (will display normally the 'gclid' value after the button add-to-cart)echo '<p>gclid value: '. $_GET['gclid'] .</p>;

The problem is certainly coming from your $gclid=$_GET['gclid'];.
You have to be sure that you are getting the value of 'gclid'.

Once you are sure you are getting 'gclid' value, you should change it a little bit, this way:

 $gclid= '&gclid=' . $_GET['gclid'];

This should work…