For the curious...
My copy of STALKER: Call of Pripyat from the old Stardock Impulse days for Digital downloads worked + activated-up on GOG.
My retail boxed DVD copy of STALKER: Clear Sky activated up on GOG just fine.
My retail boxed DVD copy of STALKER: Shadow of Chernobyl activated up on GOG just fine, also.
My Steam version's CD-Keys for Mount + Blade: Warband + With Fire & Sword did NOT activate on GOG.