I too have a recently acquired 2008 GL320 that I am using for a tow vehicle. I am only pulling a '73 Globetrotter, and right now, I am mid renovation, so it is certainly lighter than it will be once everything is back in place (I've used it as an aluminum tent a couple of times recently). Right now I'm free-balling, and do notice some sway. Been thinking about adding a simple mechanical friction sway reducing mechanism, but have been dragging my feet waiting on identifying "The Hitch" I will eventually go with.
Your total weight of 4600 seems kind of low. As I recall, my GT quoted a gross trailer weight of 4500 lbs.
I know the factory hitches on the 2007 models had a recall, requiring some additional welding. I installed a "current" model hitch on mine. I have heard of others that reinforce their factory hitch on the GL just for psychological reasons, but am reluctant to do so in the event I do more harm than good (ie. ruining the unit temper, etc.).
I guess I'll go ahead and speak blasphemy and say that I am not altogether convinced that weight distribution, let alone super high-end weight distribution, isn't overkill, at least for a tongue weight of 500 lbs. My back-of-the-napkin calculations suggest that with my expected 500 lb tongue weight, I will only be "removing" about 200 lbs of weight from my front wheels, and that is assuming I have nobody in my passenger seat, and no cargo in front of the rear axle. I'm not saying sway control isn't needed, but that can be provided with a very simple set-up.
Andy from Can-Am spoke on the VAP a few episodes ago. While I am skeptical of some of what I heard, I was interested to hear him say that with low tongue weights, the weight distribution bars with integral sway control don't do that good of a job since you don't have enough weight to really put much tension in your bars.
Anyway, there are several other forum members towing with GLs, you might do a search for permutations of "GL 320", and you will find several hits.
On a vaguely related topic...If you have changed your trailer lights to LEDs, and you haven't checked the trailer with the GL yet, you will need an adapter to put some resistance into the trailer wiring so that the vehicle recognizes there is a trailer behind it (or your lights won't work).