There are two types of GECK's. There's a in-game version and outer game which is a .esp editor, which the files Fallout 3 and TES: Oblivion were made with.
The latter makes it possible for everyone to make changes to the main files used in the retail version of the game by creating plugin's that are supported by the games data loader. This way users can dissolve glitches and add their own personal twist to the wasteland and quests. It is also possible to just make a complete new buildings and quests.
Every object used in the game (and some that were made by Bethesda, but not included in the game) are available in the GECK, but custom 3D files can also be imported and textured in the editor.