Weigand Combat sells a no drill and tap scope base for the 4" (also fits the new 4.2") and 6" GP-100 that mounts in place of the rear sight and front sight. Look here: http://www.jackweigand.com/Ruger-GP100-Scope-Mounts-No-Drill.html
Does it have a media blasted finish? If so, I wouldn't expect it to stay perfect. Shoot it, let it keep it's battle scars and age. You'll only be able to replicate it with media blasting. If it's the brushed stainless finish, then abrasive paper for touch-ups.
