It looks like the Steam version launches with Twilight of the Arnor enabled by default but I can't find an option to choose a different expansion.  I'm not talking about the campaign or anything, I just want to know how I can start a vanilla game of GalCiv2..

This is possible in Sins of a Solar Empire because the Steam launcher gives the option.  GalCiv2 doesn't seem to have anything like this.  The GC2Launch.exe file in the Steam folder doesn't do it either.

edit: Figured it out.  The executables are there, you just have to double-click them in the Steam folder.  Dread Lords is galciv2.exe in the root, Dark Avatar is GC2DarkAvatar.exe in the DarkAvatar subfolder. 

It still would be nice to have this in a Steam launcher though!