I had same issue. I have mudflaps also but didn't help much.
When new in 2019, less than 700 miles on it, I took it hunting and drove around 100 miles on gravel roads and the paint on the box in front of rear tires on the orange peel like paint " rock guard" almost looked like they had been sandblasted. The dealer was great and took care of it under warranty. In the fall of 2020 took a couple more hunting trips, maybe drove 150 miles of gravel and same thing happened along the bottom of both doors and a little bit of chipping along the on the rock guard again. Total miles on truck less than 8000. I hadn't drove any gravel since my 2019 hunting trip. Went back to the dealer and he said he saw only one other truck come in to the dealership with this issue. He had heard of a few GMCs having this chipping issue, not common to any certain color, and sounds like its rare but unfortunately I have one. They reported the issue with GM but GM said they haven't heard of this before and basically too bad for me.
My dealer is amazing. They easily could have told me it isn't their problem either, trucks over year old but they were concerned and worked with me on a solution. I ended up having them remove the rock guard on the box, had everything on bottom half of doors and front of box repainted and had them apply 3m film over those same areas. They did an excellent job and were very fair on the price. That was the best solution we could come up with besides trying to basically repaint the whole truck which would have been very spendy.