It is a little tricky. You should be able to find a map file in the GalCiv2 folder, and you should enter Data, and then CustomMaps folder.
Aka (This is the default path for installation)
C:\Program Files\Stardock\TotalGaming\GalCiv2\Data\CustomMaps
Here is guide to modding GalCiv 2
WWW LinkSuffice to say, it takes a little bit of work to make your own maps.
I hope this helps.