Steam generally sends crash reports to Stardock automatically.

However if you have a reproducable crash, you should submit a save and a crash dump to Stardock directly.

https://forums.galciv3.com/452854/page/1/

This sticky at the bottom has instructions as to what to send, where to find it, and where to send it to. Again when doing so, definitely give as much information as possible. What were you doing? Did you click a button. Was something happening during the crash? etc. more information is always important so that they can try to reproduce your problem.