I concur with Phil's assessment. This is among the most scenic bicycle rides I've ever done. The climb up Hurricane Mesa is paved and quite steep - 9-12% in many places. Once on top the road is mostly plateau country-style dirt and unobtrusive bedrock, with some gravel mixed in. It would be rather difficult when wet but otherwise the surface is good. Lots of truck ruts on the west side and cow (and bear!) tracks on the east side that added a bit of texture. The bedrock-surfaced climb from Hurricane Mesa up to Smith Mesa is steep, and the descent from Smith Mesa through Zion NP to Kolob Reservoir Rd is steep and slightly chunky. I would definitely recommend the clockwise route because you get to let loose on the Kolob Reservoir Rd descent (paved). I was running Panaracer GravelKing SK tires, 38 mm front (25 psi) and 35 mm back (35 psi) on a Canyon Endurance road bike.

Incredible views. Started at Zion River Resort. Took the hwy to Mesa Rd where the climb started. Once on top, the rollers were not bad. Took lots of pictures. Bombed down the other side on Kolob Resevoir Rd. Great ride!