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"B" is the letter in your keyboard,it's probably under G and H and between V and N.
In Gothic it opens the character screen.


Marvin is the magic word you have to type to enable the Cheats in Gothic 1 and 2.
You press "B" again to close the character screen.
If I'm not mistaken pressing "K" with enabled cheats will move your character a couple of meters in front so you will find yourself behind the gate.