Using the Galactic Civilizations II: Ultimate Edition digital download, Crossover installs the game successfully. However, the EXE it choses for the program menu doesn't work as expected. The file GC2Launch.exe should display a start-up screen with options to play the game, view the PDF manual and register the game. In Crossover, the only options are "Install" and "Readme", both of which do nothing.
Fortunately, adding the correct EXE files to the program menu seem to run the game and expansions OK.
Use the Crossover Run Command, browse the to the following EXE files and use the "Save Command to Programs Menu" button to add them one at a time.
GalCiv2.exe
/DarkAvatar/GC2DarkAvatar.exe
/Twilight/GC2TwilightOfTheArnor.exe
You may also want to edit the program menu to remove GC2Launch.exe.