Not that cool in reality.
First, Microsoft and Sony are very reluctant to allow the kind of modding that be done with the GECK on their systems.
The GECK isn't some map builder like Halo's Forge or Little Big Planet's editing system. It allows for the creation of entire worlds, scripting, creating new NPCs, items and objects (including custom models and textures), writing dialogue. Creations are also directly integrated into the game world rather than separate maps.
It uses more resources than the game itself does. Example, it requires 2GB of RAM to run. Both the Xbox360 and PS3 don't even have 1GB.
It's a Windows Architecture program. This means that it REQUIRES Windows and its attendent libraries.
The GECK (Like the Elder Scrolls Construction Set and Skyrim's Creation Kit) are the same tool the Devs used to make the game (Minus certain features such as the ability to changed things that are hardcoded and also minus tools that Bethesda licensed from third parties for their use only) They release it 'as is' to the PC community simply because they didn't have to do any additional work on it to prep it for release.
They 'could' build a GECK for the consoles, but it would be severly scaled down so that it would even run. Many of the features would be lost and it would be a wilted shadow of the current GECK. There's also that extra issue of them having to spend resources (Cash, time and people) on making a console version, so they wouldn't release it free as they'd need to recoup the costs of making it.
So it wouldn't really be that cool in the end.