There are very few bicycle tire products that can handle gravel roads. First look for phat tires that have significant tread wrap; i.e. the butyl rubber covers much of the side wall. Look also at the Armadillo from Specialized it has aramid fibers in the sidewall. Sealants only protect the tread area and won't fix sidewall slits. Same for tire liners. Lots of air pressure (maybe 10% more that rating) helps and riding on the crown of the raod where gravel is less deep also helps. As a last resort, there are solid tires, but if you buy them, also buy the tool to install them or you will kill your rims. Good luck.