OK, judging by this line in your debug.err, Steam has not downloaded all the necessary files:
Debug Message: Version 3.01, Build #10707, last updated on: 05-02-2018 16:02
Basically, your exe was not updated, and there could be others as well.
Have you excluded your steamapps folder from any anti-virus/security programs that you have running? Are you running Steam as an admin? Check both of those and then try verifying the file integrity again.
If that doesn't work, you will have to un-install GalCiv3, delete any remaining files (make sure that Steam\steamapps\appmanifest_226860.acf is deleted also) and then re-install.
If that still doesn't work, you will need to contact Steam support for further help.