Mercedes-Benz does not currently have a cargo tray available for the 2013 GL-Class that works with the third row seat folded flat. I have found that the cargo tray for the 2006-2011 M-Class fits perfectly side to side and leaves just a small gap behind the second row seats. Don’t get the mat for the 2012 or newer M-Class…it is much too wide. Contact your local parts department to order.