Complex sentence: GoldSrc (also known as Goldsource) is the retronym used internally by Valve Software in reference to the heavily modified "Quake" engine that debuted with the release of their science fiction first-person shooter video game "Half-Life".
Split into simpler sentences: GoldSrc (also known as Goldsource) is the retronym of a game engine used internally by Valve Software. It is a heavily modified "Quake" engine that debuted with the release of their science fiction first-person shooter video game "Half-Life".