Update on my end....
Turns out the leak was with the rear glass, though the dealer claims it was leaking at the driver's side even though the wet floor was on the passenger side. They pulled the glass and mentioned there was a pinched/scuffed seal. Not exactly sure if they just replaced the seal or replaced the seal + rear glass. Never got any paperwork when picking up the truck as the ticket was still open for other issues, described below.
In order to fix the rear glass, they apparently had to pull the entire bed off the truck. Seemed extreme, but it seems like the leak is fixed. We had a ton of rain here today and everything is dry so far (knock on wood).
The first bad news was that they scratched an interior trim panel while reassembling the truck. Additionally, they pointed out that the gas tank filler neck plastic shroud was all busted up. My guess is they actually broke this while pulling the bed (probably forgot to disconnect the shroud from the bed), but they didn't exactly own up to it. They did tell me they had both parts on order when I picked the truck up though. I'm guessing this is why they didn't have any paperwork for me as the ticket is still technically open.
Then things got worse. Went next door to gas up after picking up the truck, and immediately noticed a nice dent in my 3 week old hard folding tonneau cover. Drove it right back to the dealership to show it to them before they closed.
A little later, during the first wash since picking the truck up, the driver's side plastic roof trim popped completely loose from mid roof to the rear glass. I was able to reinstall it, but immediately noticed that the trim was bowed heavily and wouldn't lay flat. Another casualty from replacing the glass I guess.
So if anybody is keeping score...
Fixed = rear glass water leak (so far)
Broken while at dealership = scratched interior panel, broken gas tank filler neck shroud, dented tonneau cover, and plastic roof trim piece seems damaged
The interior panel and filler neck shroud are on order supposedly, so that's good. It seems like they are going to replace the dented tonneau, though that's not officially been confirmed. No word yet on the roof trim as they have only seen the pic I sent them.
I know sh$% happens, and the dealer seems to be taking strides to make things right (or at least fix what they broke), but this has been painful to say the least. It's been over 3 weeks since I first reported the water leak, and I still have to take the truck in to get the remaining issues fixed that were caused/broken while the truck was at the dealer. My confidence in GMC service is quickly eroding considering I've had two serious issues with the truck and it barely has 2000 miles on the clock.