If you hadn't turned off Steam cloud sync during the beta, before last Thursday, you're probably getting bit by that.  It had been causing problems so for 1.0 Stardock turned it off, but Steam is still doing it, and in another thread they said that Stardock is currently working with Valve to figure out what is going on.

It's not turning off Steam entirely, it's just turning off the Steam cloud sync feature, which copies your saved games and other local data files to its cloud.  You used to be able (during beta) to turn it off for GC3 by selecting GC3 in Steam, right clicking, selecting properties, and then someplace there was an option to turn it off.  Now, since 1.0, that doesn't work.  I've seen other people report that you can turn it off globally for all your Steam games, but I'm not sure how to do that.  Unfortunately I'm at work now and can't play around with Steam to figure it out.

Found on another thread, here's how to turn off Steam cloud sync entirely:

No, it won't affect your updates, and don't disable Steam.  All this will do is to prevent Steam from uploading your local saves, designs, races, etc to the cloud and then restoring them from the cloud if you delete them.  It doesn't affect the game files or updates/patches at all.