I think the GM tailgate is a total gimmick. I've owned pickup trucks as my daily driver for 30-ish years now. I think if I had had that GM tailgate through all that time, I might have actually used any of its features maybe half a dozen times. And probably would have effed it up on a trailer hitch, too.
OTOH, I think the barn doors are something I would have used many, many times. I very rarely drive with the tailgate down. Generally only when I have a motorcycle in the back - which is very rare itself, because I generally ride there. My bikes are not trailer queens.
And if I'm not going to put something on the tailgate and leave it there while I'm driving, why do I want the tailgate laying down in front of me when I'm putting stuff in and out or getting in and out of the back?
Most of what I am putting in and out of the back on a regular basis is scuba tanks. I generally put them in laying down at the back of the bed. With a regular tailgate, it is just in the way and makes it that much harder to put them in and take them out because I'm having to lean across it. With single tanks, it's not that big a deal. They generally weigh around 45 - 50 pounds. But, I weighed my doubles the other day after they were filled with air. 99 pounds. Double steel HP120s, if you know anything about scuba and care. Picking them up to to put them in the back of the truck is not a big deal. But, then getting them across the tailgate and into the bed, so I can close the tailgate is a minor pain. It would just be a lot easier and more convenient with barn doors, so I could set them in just where they need to go.
Scuba tanks. Bags of concrete mix. Bags of mulch. Boxes of "stuff". Whatever. Anything that you're putting in the back of your truck and then going to close the tailgate would be easier with barn doors.
In other words, I think I would probably use the barn doors about 95% of the time, versus opening the tailgate the "normal" way... if I had the new tailgate.... And with the barn doors open, the new lower middle part of the bumper means I would not need any kind of additional step to get in and out easily.