I installed this adjustable sight on my GSG:
http://www.midwayusa.com/product/721...cut-steel-blue
It does take a bit of fitting. While the angle of the cut is correct, the depth, front to back of the bevel is longer that the stock sight. I shaved down the front bevel with a file & sandpaper. Took a while. That will get the sight to fit, but there is another problem beyond that. The metal where the rear sight mounts is very thin. The stock sight of the GSG has a plate under the set screw that distributes the force of the set screw. Without the plate, the set screw can dent the metal causing slide problems.
I milled out a circle under the new sight so I could install a plate to do the force distribution. How handy are you with tools?
I have not seen a "plug in" rear sight. Also note that the rear sight I chose DOES NOT HAVE DOTS.
Was all the work worth it - yes. The adjustment via the 3 heights of front sights did not work for me. I am using the middle height sight with the SA rear adjustable.