I have a couple of hundred miles on these tires, so this is really just a first impressions review.
Even though these are tubeless ready, I intended to run them with tubes. I moved from New Mexico to Florida and no longer fight goat-heads on every ride.
I had been running Hutchinson Sector 28's on a ghetto tubeless setup and they were great tires while they lasted, both the initial and warranty replacement Sectors had the sidewall separate from the bead. I thought that the Sectors were the most difficult tire to mount, but the Gravel Kings gave edged them out.
I ended up removing the tubes and mounting tubeless to give me the bit more space needed to get the Gravel Kings mounted. It was still a struggle even with a Tire Bead Jack. I imagine that these are generally a tight mounting tire, but they would probably go on much easier with a true tubeless rim. I'm on a budget)
While not as fast as my Sectors, moving to a 38 Gravel King makes my Cross Bike ride extraordinarily comfortable. I can keep my butt on the seat over terrain, like uneven sidewalks, that would previously have forced me to either stand off of my seat or visit the proctologist. The ride is acceptably fast for a tire with the comfort and traction offered by the Gravel King with acceptably low rolling resistance.
In Florida I deal with a lot of blown sand and I comfortable attack my ride through such obstacles. That said, this will not work, nor did I expect such, in deeper sand.
Durability is still out, but I've run over a bunch of stuff strewn into the bike lane and steered off road to avoid oncoming bikes and pedestrians without issue in the brief period that I've been running these tires. I've yet to get sprayed with Stan's tire sealant, not even small punctures to this point. That's OK, I had a virtual Stan's shower on the last few miles of my Sectors.
Early verdict is that I would highly recommend these for someone seeking a comfortable and secure ride for road and light off-road riding. Mounting is the only downside and that issue is likely less of a problem for other rims. I'm never going back to 28s or smaller unless I have a second dedicated road bike.
Thank you for your feedback.