Front sight: The GP100 (at least the adjustable sight version) comes with an interchangeable front sight. Some S&Ws come with interchangeable sights, but most don't. Though they can be converted, it'd take some milling by a gunsmith (read: time & $$) to accept the Weigand base.
While this is true of the 4" and 6" barrel models, it bears mentioning that the 3" GP100 does not have the interchangeable front sight.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar