Some years ago, I set out to bicycle every official county road in Weld County, CO. I only got ~600 of the 3000 miles done before I moved and a majority of them were gravel roads.
So I looked through the detailed maps to see how they crossed Northern Colorado. My sense is for at least northern CO and southern NE this is essentially a picking of a particular set of gravel roads - where often there is a network grid - often alternate county roads a few miles to east or west. So even without the official Great Plains Trail listing, one could also ride at least those areas with a more detailed CO map e.g. Delorme and just pick county back roads and expect mostly gravel riding. Not a big issue unless one gets a combination of rains along with poorly graded road that becomes a mess (not all gravel roads are a mess in the rain, but Weld County at least had a few and I've seen similar in other states).
As far as the appeal of that, it might appeal to someone who has started riding all county roads in Weld County or the length of US83
You can expect some wide open spaces, a need to plan routes to include occasional services and except for National Grassland, mostly private property that started originally as marginal homestead lands but since has largely consolidated into large acreages. This is still a good amount west of the 100th meridian, a traditional dividing line between areas getting 20 inches a year and those receiving less with a lot of impact on farming, etc. I would suggest riding south to north due to prevailing winds - not always in your favor but more often from SW / S than from the N.