Can't find the reason why and can'g get a response from tech support. Don't know what to say.
Whismerhill and I both advised to try setting the resolution manually here: https://forums.galciv2.com/330927
If that does not work and you have a support ticket, post your ticket ID and I'll have them pull it up today.
But after reading this, and the fact that I have a store bought-installed Gold version installed as well, I really have to wonder.
The OP's issue was a simple one--the GC2 Gold installer is old and does not write all the registry entries that Impulse needs. Reinstalling via Impulse is the simple solution for those who don't care to muck about in the registry by hand. If you're unable to do the extra downloading, support@stardock.com can assist with the registry fix.
Stevepond's issue is something else and is unrelated to this.