Five parks up the middle of the country, and two of them are not really Plains.
Badlands, Wind Cave and Theodore Roosevelt are the real core: eroded grassland and prairie across the Dakotas, close enough together to do in one trip, and criminally under-visited compared to what they deliver. Wind Cave protects one of the longest and most complex caves on earth and the prairie above it in the same park.
This is the smallest region and the easiest one to finish. Three of the five are within a few hours of each other.
Voyageurs is boreal lake country on the Canadian border, which is Northwoods, not Plains. Gateway Arch is a 630-foot monument in downtown St. Louis and the smallest national park in the system. Neither is Great Plains on any strict reading. They are here because the alternatives were worse, and because a five-park region is more useful to someone planning a trip than two orphans and a three-park region.