Checkmate, First you must have a GP-100 with the "plug-in" front sight. These are standard in all GP-100s with adjustable rear sights. Fixed sight models have a pinned front sight ... not a plug-in.
Note the small hole above the muzzle that goes back into the barrel's rib. Use a punch or other tool to push the plunger to the rear. At the same time, pull up on the front of the sight. It will pop out when clear of the channel. To install ... place the sight on the rib with the extension in the channel. Hold the sight to the rear and push the plunger in. The sight will snap into position.
I would take it to a good gunsmith and get it done right the frist time. Now remember I said good gunsmith not some metal butcher. Talk to other people at the range, shows, and stores. All of a sudden you will start hearing the same guys name over and over. He's the one you want. Good luck.
Baldy, Maybe you aren't familiar with Ruger's plug-in front sights. They are designed to be changed very quickly by the owner without need for special tools. It takes about 15 seconds to swap my factory black front sight for a red insert. If you need a gunsmith to change the plug-in sight, you would also need a gunsmith to load or unload the gun.