Gclid is a parameter tracking is unique in the world is (ID, click Google) that for the transfer of information from Google AdWords and Google Analytics Google can be used. If you tag this, auto url's in Google AdWords active. Google is a parameter unique gclid in the URL, your destination at run time adds. Because of this, a change the route (Redirect). you have no parameter gclid in the target URL will not see, but in the Log the incoming server you will be shown.
parameter gclid possible to share data between Google Analytics and Google AdWords provides.