To disable the hud in Gothic 3, I recommend you first download the updated community patch version 1.75 or later which is available from a variety of sources online. However, that isn't necessary to disable the hud.
To disable the hud, first:
Quickly type: marvin
Then, press: Ctrl + F1
That will disable the hud.
Press Ctrl + F1 again to bring the hud back.
This must be done every time you start the game. The setting will not hold.