I’ve been trying to trade a pre-lock S&W 66 for a post-lock 686 because since about 93 it is tapped for a scope mount from the factory. I’m a S&W guy and not real familiar with the Ruger revolvers, is the GP100 tapped for a scope mount?
However the rear sight (if adjustable ) can be removed, and a Weigan mount can be installed for a optic, if that helps.
No mods to the frame.
This is the short one, but he also makes a full length version for 6” barrels
As noted, no the GP-100 is not drilled and tapped for scope mounts.
However, Jack Weigand sells a full length Picatinny rail that mounts in place of the rear sight (after the sight is removed) and in place of the front sight (again, after the sight is removed). It is available in models for both 6" and 4/4.2" barrels and installation is very easy. Its very strong and secure but a bit bulky and heavy.
Weigand also makes a short Picatinny rail that mounts only in place of the removed rear sight which is a lot smaller and lighter but not quite as strong. It should be adequate for even .357 use. I have one of these on my GP-100.
Finally, Evolution Gun Works (EGW) makes GP-100 mounts for a variety of dot sights (Vortex Venom/Viper, Burris Fastfire 3, Leupold Delta Point Pro, etc., etc.) that also replaces the rear sight. I've uised these on several guns and they also work very well.