I'll second (or third, fourth...?) the comments about tires being the main limitation.
Not an issue at all in dry weather. I ride my R100RT on unimproved roads fairly often, and I've also been riding my 1965 R69S offroad quite a bit lately.
In my opinion, the Earles Fork models are better than R100GS's on gravel roads where ground clearance isn't an issue.
I left the guy with this GS for dead on the washboard and deep gravel we found on this road.