I wonder why Stardock support does not simply post the Windows8 workaround in this forum?
Here is the solution I received from Stardock support, working fine:
Quoting Stardock support,
This is a workaround activation fix for Windows 8.
You should only have to do this once, unless you re-install on this PC or another:
Download: http://sd.stardock.com/sdactivate_legacy.zip
Please extract its contents here:
C:\Program Files (x86)\Steam\steamapps\common\galactic civilizations ii - ultimate edition
If it asks you to overwrite the existing files say yes to all.
Run the REGISTER_GC2ULTIMATE.CMD file.
Enter your account information and serial key when prompted.
That should activate your game and get you going.