I have a 2014 diesel GC, and love 95% of it.
I tow a ~3600 lb TrailManor, which is an expandable, so it tows like a popup. My only trips towing the TM have all been round-trips from Denver into the mountains - Telluride, Grand Junction, Fairplay. My round-trip fuel economy while towing is 21.5 mpg on every one of those trips. Without the TM, same trips are about 29 mpg.
Goes great on the flats and uphill. Not lacking for power (again, I am not towing a barn, though). I have gone off-road with it, and it does great in low-range, crawling up or down the hills, yes, while towing my TM.
My only dislike is towing downhill. Diesels, by their nature, are poor at engine-braking. So, I have to tap the brakes regularly going downhill. Pickups can get exhaust brakes, and they don't have to use the brake pedal on downhills. Just be aware that the diesel won't hold you back on downgrades.
Trailer Life had a
great article using the diesel GC to tow a full-height ultralite. They got 12.4 mpg towing the barn. They were impressed with the GC. It is a good read.