My "gravel riding" is limited to 1 - 5km passages over gravel sections of road, linking up paved roads, or perhaps a bit further along well graded rail-trails. I do this all on my road bike with 700 x 25 tires. For the most part these gravel roads are what I call "good gravel" in fact some of them are smoother and in better shape, than some of our local, frost-heaved, cracked, pot-holed and badly patched paved roads!
Occasionally I will encounter rougher sections - usually wash-board in the middle of the road, where most of the car traffic goes. I've often found relief and a smoother ride, but going close to the edges of the road and seeking out that ribbon of loose gravel, between where the cars have been going, and the actual edge of the road. There is still a firm under-section, with a bit of loose gravel on top and it's often WAY smoother than the washboard in the tire-track sections!
Steve Fleck @stevefleck | Blog