According to research by one of our users - Norgur8 - some cards treat Gothic 2 as a 2D application, and therefore run it in an extremely downclocked (energy efficient) state. This in turn causes poor performance.
To get around this, create a Catalyst Control Center profile for the game and force some hardware-intensive graphical settings, to make your card "work harder".
Right-click your desktop and launch Catalyst Control Center. Afterwards, go here:
Gaming tab -> 3D Application Settings -> Add profile for Gothic 2
You should enable settings such as Anisotropic Filtering and Anti-Aliasing. You can set them as high as you want, and as a side-effect the game will look a lot better.
Click here for an example of how you can set this up.