Open the ge3.ini file in the Gothic3/Ini folder and change the following line in Notepad:
[Game]
TestMode=false

to

[Game]
TestMode=true

You can then press [~] (tilde) while playing to bring up the console and type any of the following cheat codes.

Code:--> Result:
God--> God Mode
Invisibility--> Invisibility Mode
Teach all--> Learn All Skills (lowers stats to 0)
Kill--> Kills Target
Give--> Give Specified Item
Spawn--> Spawn Specified Item/Creature