The tailgate will be an option so no need to tie your crotchless panties in knots. The trucks they roll out in those presentations always have every gimmick the designers come up with. That's why the one shown is a Denali and an AT4 (AT4? Lol, like your going to go beating a 70K pickup to pieces in the woods), which will be the top of the line, special edition, $70K ohhhh look at what I have that's nicer than yours model to show off at the gas station, pedicure parlor, or grocery store. The badging will vary between trim models and special additions.
They will not have that tailgate on the bottom of the line Sierra (The twin of the Silverado WT/LS, the contractor version) and it will be optional on the SLE, most likely part of a convenience group package.
Looks like they went with the every other generation body panel difference between it and the Chevy with different Fenders and quarters.
The roof line sloping even lower at the rear than the current generation, and the bump out at the rear of the cab assures your not putting a bed cap off a previous generation truck on this one, and to match cab height you'll lose headroom in the cap.
When the led taillights and running lights **** out, be ready for sticker shock on the replacements.
For those that comment what the tailgate would cost to replace, that's insignificant. A deer hit on my 15 GMC last week, which dented only the steel/chrome part of the front bumper and plastic center insert was $2K. A previous slam that creamed the entire bumper assembly (100 parts and brackets), grille, fender, and headlight assembly, was $10K.