Funny, I've had some non-tubeless 28c GKs and they've been great. Only one flat in, I don't know, 4000 miles of riding. Admittedly, it was just a thorn, so maybe I just have a nice bike route. Usually the debris causing flats are screws, nails, or staples that would go through almost any tire.

As for looser surfaces, they are called Gravel Kings. I've done some rides on them that were much more suited for mountain bikes. They were fine on smooth and slightly gravelly double track.

My problems on single track were probably more my lack of handling skills- riding a bit too high up on the edge of the path and losing traction. Beat going off the other edge down the incline.

Edit to add: my only issues on double track were problems with traction on moderately loose but pretty steep sections- stuff where I was having problems climbing in my 34/28 low gear.