The Creation engine is built from the Oblivion engine (please don't call it "the Gamebryo engine," strictly speaking it's not an engine). They completely rewrote the renderer, AI, pathing, thread handling, the way it loads data, basically everything that makes an engine an engine. They also replaced a number of middleware programs like SpeedTree with their own stuff. It's new in the same sense that Source was a new engine, despite being built from GoldSrc.