Fifeldor: OK, I managed to get the game to work.
For whatever reason it was not working with Windows 8 compatibility mode, so I set it in Windows 7 compatibility and would still not work. As soon as I set it again in Windows 8, it worked flawlessly.
I then installed the 1.74 community patch and use ra 1.75 ge.ini file to fix the DOF, but text and sound would not work. Thankfully I remembered there was a 1.75 community patch released as well, so after re-installing the game, everything works flawlessly.
For future reference, GOG integrated version 1.74 of the community patch into their installer. If you re-install Gothic 3 again you will only need to install version 1.75 of the community patch.