I had a few game's here in gog and steam too.
I wan't to add Defcon to my steam library as an original game.
But I'm not able to do that, cause it's says " invaild key "

Any ideas?

Try doing the following:

Once you are logged into Gog.com, do the following:

My Account -> My Games -> Defcon
Highlight the code underneath My Serial Number and press ctrl+c

Now activate Steam
Click Games
Click Activate a Product on Steam...
Click Next
Click I Agree
ctrl+v to paste your code in the Product Code box
Click Next

If that works, have fun. Otherwise the Gog code does not work on Steam

PF