Quaker, You can change sights on a standard GP100, but I don't think you can change out to the rear sight to the Novack sight like the Wiley Clapp model. It looks like you could change the front sight to the Wiley Clapp fiber optic style.
The more I look at it, the more I like it.
A number of years ago I saw a picture of an SP 101 that had been customized with a Novack style rear sight if I remember correctly. I think one of Bowen's employees did it for himself. Sure was sweet.