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.
You have a lot of good options.