How?

How are you supposed to open it??

Acording to the add-on readme file, it is done via command-line (Ex: In Windows you would tack on "-mod DARKCONS" after the "../gc.exe" in the properties section of the shortcut) but I can't figure out how do that with Ubuntu.

Thank you.

So, GC-DC can't run in Ubuntu then?

A new Mint user here. I haven't tried the game yet but, so far, all the windows games I have were played through PlayOnLinux.
I've observed that when configuring a game through the Panel, you can see in the General Tab a space to put arguments.
So, you could try this and came back if works.