Transmission Failure problems of the 2015 GMC Sierra

2015 GMC Sierra 6-speed automatic transmission failure @ 1800 miles. As I was decelerating from approximately 30 mph to make a turn into a road the transmission downshifted as expected to low gear. As I accelerated the transmission shifted to 2nd normally. At this point the transmission should have shifted into 3rd gear but the tachometer jumped to 3000rpm before I realized that the transmission had shifted into what seemed like neutral. This left me blocking traffic with a truck that would not go over 20 mph. When I slowed sufficiently the transmission would pick up 2nd gear again. Being close to home I was able to limp home. When I reached home I past my driveway and went to back in as I usually do. The transmission would not pick up reverse! by this time a check engine light came on. I then had to drive about a mile down the road to find a place to make a u turn so I could pull it into my driveway. The truck was towed to the dealership where a mechanic experienced the same problem on a test drive. (he had to push the truck backwards by hand to be able to return to the shop. ) currently the transmission has been removed from my brand new truck pending investigation. It has only to gears 1st and 2nd. This is a serious failure that could have happened while merging into traffic on the interstate. Luckily I was on a back road in the middle of winter!.