Hi, I have Gathering of Power in my downloads on this site and I also bought the complete pack on Amazon during a sale two X-mas's back.My copy of Spectromancer on Steam is missing GOP however. How do we get the serial for Steam?
...You can use your GoP activation code in any game copy regardless of where it came from (Steam or not).
So I can just install the base game from Steam > Enter the GoP code and it should add the DLC to my Steam library?
...No, the game can't change your Steam library. But you said you bought codes from Amazon and AFAIK Amazon doesn't sell game codes - they sell serials for Steam. Such codes are used in Steam and add items to the Steam library.