I keep noticing that many Linux issue threads have one thing in common: Missing libraries.
A while ago I've uploaded a patch to help solve a few of these issues which you can find here: witcher_2_libraries_linux_mint.tar.gz
This package will make the game run out of the box on Linux Mint without requiring you to install additional dependencies. It should work for other distros too.
1) Download the file from the link above.
2) Extract the contents in your "Witcher 2/game" folder.
You're now ready to run the game.
Notice: Please be aware that while GOG may not always provide dependencies right out of the box they do list them on the right hand side of the game page (see attached picture below for reference).