My GF has a 26mm GK on the back of her bike. It's done just under 4000 miles with no punctures in year round riding.
Most of the use has been on-road with some gravel (usually when I choose the route .....), day rides and credit card touring.
The tyre doesn't have any cuts and the only thing that gives its age away is that the tread is squared off..
GF hasn't commented on either lack of grip or speed.
When I fitted the tyre, it went on very easily, something that I regards as a major bonus should the tyre need to be "fixed" at the side of the road.
Having said all that, I think she's very kind to tyres; the 25mm Schwalbe One on the front of the bike has done ~2500 miles. It looks like new and again, no punctures.
For the price, I reckon the Gravelking is a real bargain.
Based on this experience (and some favourable reviews), I've just fitted a 32mm Gravelking SK to the rear of my bike. It's too early to tell (only 25miles) but it seems to roll well. It'll be used year round on the same type of rides as above but it'll also get a lot more gravel bashing.
I'll report back.
If all goes according to plan, I'll get another GK SK for the front wheel to replace the 28mm Marathon Supreme that's on there ATM.
That's another great tyre IMO......