Are people having luck with 700x38 Gravelking SK's with Mud Butlers? Seems like it would be a problem unless they measure same as 700x36 MSOs.
You'll be able to clear the 38mm SK in the front, but probably not the rear. I cleared a set of Rene Herse (Compass) Bon Jon's on my Expat on a set of American Classic Race wheels that measured 38mm inflated with Mud Butlers, but it's a slick tire. The tightest point will be at the rear seatstay bridge. I was able to gain a bit more clearance by putting the fender mounting bracket on the underside of the seatstay bridge and gently bending out the sides of the fenders with a crescent wrench. In the front, tire clearance is better and you should be able to clear the SK 38 no problem. I've got a 38mm G-One that measure's 40mm on the front right now. On the front wheel the tight spot is going to be at the mounting point where the strut attaches to the fender and may require some manipulation as well. Here's a pic of the rear bracket and the sort of clearance I have with the G-One 35 in the rear. You're going to need to get some real world measurements of the Gravel King's before you make any decisions. You could always order one 38 for the front, test it out in the rear and if it doesn't fit run a 35 instead.
