I have only owned or hauled BP trailers, because they've been deals too good to pass up. I have messed the weight distribution up on the 18' dovetail, and got lucky to get out intact. The nice thing about GN is that they always are planting a share of the weight between the truck axles. In application they should be much more forgiving for varied loads and load configurations. I don't know that you can load a GN to lever up the rear truck axle the same way a BP can and start getting driven by the load (?).
If you're buying anyway, and keeping the car hauler, my .02 would be to listen to whatever got you thinking GN, and give yourself a different option. Heavy loads and bad roads can create some pretty dynamic weight moments when the load seriously outweighs the tow vehicle, which I'm guessing yours does.
I still shudder when I recall seeing the tractor darting in and out of each side of the rear view mirror after hitting a huge pothole on a steep sweeping corner. I'm lucky I live in the sticks, no one was on the road, and a friend had described his Dad's crash when the same thing happened some time earlier. I punched it, and was able to get enough pull to tug the tongue back down. The first instinct was to go for the truck brakes (didn't have one set up for the trailer--then!), which would have been a quick wreck. Very fortunate to get the education without incident.