GED is included in Gamestudio and you find it in your Gamestudio folder. It is a ingame level editor and useful if you only use models and no WED blocks because GED can only display them but you can`t edit WED block geometry. GED was created by the developer "crew51". They also developed the game "Glider - collect n kill", a very polished and good game made with Gamestudio. Unfortunatly GED has never been free of bugs.. it is very unstable. I would call it beta. They discontinued it very early... but it`s usable if you build your game from stretch in GED I guess.