It's hard to say what things are exactly, without being able to see it. There's also a lot more integration between the transmission and the engine control on the newer trucks with the 6-speed autos. So, it may still just be shifting, though it might feel a bit different than you're used to with the older series of engines/transmissions.
If it's a brand new truck though, I do wonder if maybe GM has done something to their 2013 vehicles that makes them a bit off. I test drove a new Sonic recently with a 6 speed auto in it, and the transmission was behaving very oddly driving around town. I noticed that it would shift up/down between 2 gears a lot just holding my foot steady at ~45-50 MPH, and the shifting felt very odd when accelerating to speed, and when slowing to stop. I repeatedly mentioned the weird behavior to the salesman in the car with me, but I don't think they did anything about it, and the car was sold only a few days after that, and I never noticed it moved from where it was parked for display, until it was sold and I saw the new owner driving it on the road. It may have been a simply issue in the programming, and I wonder if perhaps the same or similar issue has happened with the truck you're discussing here as well.
Its not my truck, I was only doing a favor by bringing it to the dealership for my friend since she works so much, so I won't be able to get that video for you..
Just something to try ?? I read were someone was having similar troubles and they found that the traction control needed reset.. All they did was turn it off and on a few times while driving at about 30 M-Hr ?? Can't hurt anything to give it a try ?? Good Luck !!