So, how does one do a roan on gimp? Any brushes, or just overall ideas on how to?

i usually outline the horse's body with the lasso tool then i use Chalk 02

Any other suggestions?

Select the area of the horse that you want roaned, and I usually lighten and desaturate. I follow up by darkening the areas that need shadows with the burn tool, and dodge the highlights. I do my roan similarly to how Wings explained.