Fixed. As a software engineer I find it a bad idea in general to add a string to a system variable which has a max size limit that any program/company can add to. In my case AMD and Nvidia had some long strings in PATH. Thanks for the effort to fix this.
I have the same problem. Launch GalCiv3, the cursor spins for a few seconds then nothing. Same result using the Play button inside steam. Check Task Manager and GalCiv3 doesn't show up in process list. If in steam i right click GalCiv3, select properties, click Local Files tab, click verify integrity of game cache, it finds 6 files that it downloads. It finds them everytime. Those 6 files are SteamApps/common/Galactic Civilizations III/data/