The post in that thread about a corrupt file happened to me.
I got GSB as part of the Humble Bundle (in some ways the reason I got the bundle), install the Mac version from Steam and ran it. At the time it was version Full 1.55, next time I ran GSB it said thee was an update available and so i let it install it (though was surprised that the update didn’t actually come via Steam) after which the game was 1.56.1
In searching around to find out what the update was/did I found this thread, read the linked thread and ran the cache validation and found the corrupt file.
Now was the file actually corrupt or was it simply a new signature from the 1,56.1 update that Steam doesn’t know about?