I was going to get some GK slick 38's, but when I needed them this past spring, there weren't any in the U.S. to buy. So I bought some Maxxis 40mm Velocita. Seems about the same as the GK. I run GK 32mm slicks on my road bike. My only complaint is that I wear through the treads fairly quickly. I have come to the conclusion that the Rene Herse tires might be a better value from a tread wear consideration since they have twice as much tread.
Central Pennsylvania gravel is pretty nasty, I only ride the GK SK on that. I don't consider them to be slow tires on pavement.