There are more than a few of us towing larger Airstream trailers with our GL's with excellent results. Search this forum for Mercedes GL and you will find a bunch of threads on this. Rich Luhr, who writes / edits airstream life magazine also uses the GL as his tow vehicle.
Assuming you already know about staying within the overall towing capacity, the bigger issue with the GL is going to be tongue weight - you will likely need to get your hitch receiver reinforced.
There are also good threads on towing on the Mercedes GL forum at GL Class (X164) - MBWorld.org Forums
Good luck, and happy to help if you have more specific questions once you've reviewed the threads above.