I just finished a game of Gothic a couple of weeks ago. As I recall, I switched the control configuration to Alternate, then went through the list key by key and rebound everything to my preference. I hit Delete or Backspace (I don't remember which) to clear each setting, then Return followed by the key I wanted.

To rebind keys to mouse buttons I hit Return followed by the mouse button I wanted. I didn't have a problem with the game recognizing the mouse. I have a four-button trackball, and I had no trouble rebinding keys to buttons 3 and 4 either.

To skip the voice acting, I would click the Right Mouse Button.

I suggest setting the configuration to Alternate to begin with. I believe that by itself makes Mouse Button 1 the action key.

Make sure the control is set to Keyboard and Mouse and not Keyboard only.

In Controls configuration, chose Alternative Settings, and this should do.
Even if you do not see mouse there, it should work by default, that is M1 + W.

thank you both, it worked for both
problem was, the keybinds menu doesn't display the mouse button if you set it to anything, the field stays empty like an unassigned key, but it works normally in-game. I thought it didn't register it at all

I didn't say anything because my reply would not have helped but, now that you got it working, here's something (if you can do this) you can try that'll make it easier still: program your keyboard to do two keystrokes in a single stroke of your choice so there's no need to hit both keys to do common things. I think you can do that with a gaming mouse too.