I've ridden on gravel some with my Vittoria Rando Pro's (1.5") they did just fine, but I imagine pretty much any slick that size would perform about the same. I did find them a little slippery on wet grass on a slope, but again, I think that would've been my experience with any slick 1.5" tire under those conditions.

Slick of course meaning largely tread free. Most of these tires have some vestigial tread, but it is pretty much just for show.