Is that even posible? I talked with a friend of mine who is really into the Fallouts and when I mentioned that the Navmesh bug had been fixed for Skyrim and he wondered if that could even be possible for the GECKs?

Bethesda are done with Fallout. The whole mod community there is built around MasterUpdate patches and ESMifying stuff and has been since release.

The NavMesh bug only got fixed in Skyrim because everyone absolutely played hell about it.

But couldnt the fix somehow be implemented by modders? I pla to play the Fallouts after I am done with Skyrim...I am not a fan of player houses or whatever but just asking

Nope. It's an issue with the game engine/editor executables themselves (namely how Navmeshes are saved with an incorrect Mod Index in the editor, and then how the engine deals with that). It's not something the community can fix, there are workarounds, but no fixes.

Ah well I thought that once th fix had been implemented in Skyrim maybe it could be somehow reverse enginered to the other games that suffered this

Beth would have to do it, but they wouldn't. No game company would. Fallout is an old product and it's no longer supported anymore. They can't continue patching old games or they would have no time to make/fix the new ones.