Check how much your memory your page file (virtual memory) is set. I experienced crashes and unable to load Immense maps and crashes on Gigantic till I did the following:
Reinstalled the the 'redistributable' which stardock support told me to fix, here is thier email to me...
The crashing looks to be related to the Visual C++ Runtime library. Please navigate to the following folder: C:\Program Files (x86)\Steam\SteamApps\common\Galactic Civilizations III\_CommonRedist\vcredist\2012
Run the 'vcredist_x64.exe' file within there to reinstall the redist. Once that has been done, try to play the game again.
Please let me know if this does not resolve the issue.
I also increased my page file to 6 gigs fixed. This immediately solved the 'unable to load' problem.
Both of these 'fixes' will not harm your computer or your game. So its worth a shot to see if they work.