MrMagniloquent The installation path was root, because you executed the installer with root privileges (using sudo).
When you execute the installer as a normal user, the default path is in /home/.
First of all, install the game as a normal user, maybe then DLC will work?
As kyrios said, never run GOG installers as root. Also, never run WINE as root (it's never needed anyways), as things are likely to break.
Also, you need to know that GOGs Linux support is pretty lackluster when you compare it to Steam. Updates missing, stuff left broken, issues not present for Steam users etc. Not all games on GOG are like that, but some of them are.