If you are the owner of retail or Steam versions of STALKER, you can now reclaim them on GOG, free of charge and without any DRM.
All you have to do is enter a valid CD KEY here:
Games will instantly appear in your GOG library.
You can read more about it here:
I love GOG and their DRM Free policy. So I can be assured games are really my games and not just leased until the service goes south.