A Modding Guide
https://www.galciv2.com/mods/gc2modsdoc.htm
from
GalCiv2 Forums
Along with the code changes that I made for 1.2, I also made a guide to help with modding galciv2. It covers all of the data files that you can mod in GalCiv2, although it occurs to me now that I did not cover screens. However, there are docs out there for DesktopX, and those would cover most of what you need to know.
Anyway, have fun, and please e-mail feedback on the modding docs to [email protected].