Here's what I've done. One trip to the Pacific coast north of Cambria, including about 15 miles of progressively rougher dirt road, ending in a few miles with rocky creek crossings every quarter mile or so. Then along the coast highway and up a dirt road to the ridge, camp for the night, then along the ridge--about 25miles in all of decent dirt road. On this trip I discovered the GS8 has a lot of torque in second gear, making the sensitive throttle much less annoying than in first; the bike tracks straight and true across the rocks if you just stay with the program and don't freak out. After I let the tires down to 25 psi or so the ride was better in the rocks.
One weekend trip of 280 miles in SoCal, including 70 miles of dirt roads from well graded to more rocky creek crossings and many rocks in the road. Rarely used first, was able to plonk along in 2nd down to 1500 rpm or so. Most of the roads were in pretty good shape, with some soft corners and the occasional rut. Camped on a ridge again, watched another great sunset over the Pacific. The bike is doing everything I want it to so far, though I know there are greater challenges ahead.
My assessment is that it will take me to and around the White Rim Trail in Canyonlonds without a problem, unless I geek in the sandy sections.