stevinator,
That that "dowel or pin" is the plunger that holds the front sight in.
Press in on the spring loaded plunger and lift the front sight off, starting at the rear.
The sight may, or may not, come right off easily. It might require a little leverage under the rear portion of the sight.
I would say "refer to the GP100 manual" but it doesn't give instructions for removing the front sight that I can find.
If you need more info let me know and I'll post pictures.