They redoodled the sizing on Gravel Kings so they are more true to size. Are your tires fairly recent?
I just picked up the Velocitas on the recommendation of my LBS, so I didn't do any research. Needed a slick tubeless 40mm and that's what they had.
I think there was a problem getting slick GK at that time. It was when they were introducing the + casing for one thing.
Just don't get the refuse. I refuse to use a tire named after trash.