As for the fat bike, there is no such thing as a gravel road that will stop one. You can definitely do gravel with a fat bike and do it in great comfort. I do it all the time. I actually ride my fat bike all year long, even out on pavement, though mostly on the unpaved shoulder along the pavement. (Definitely get some looks from motorists. ) Those big lugged fat bike tires for snow or off trail riding will drive you nuts on a paved surface, though, and they sound like a tractor. The good news is that there are fat bike tires with shallower, less aggressive tread and I may opt for these for my summer fat bike riding. Would be ideal for gravel riding. Some of our local fat bike riders actually prefer these shallower tread tires for riding in the sand because they don't dig down in the sand as much as the heavy tread tires.

For sure, fat bikes are much more than bikes for riding in the snow. The only drawback for me is that these aren't really a good choice for doing a lot of miles at a decent speed. Ten miles on a fat bike is a real workout compared to a road bike.