Garry Newman placed this error into the game and set it so that only people who pirated the game would get the error.  He posted this message on his Twitter today:
"Just enabled this error in GMod today. It happens when you pirated it. Having fun watching people complain. http://goo.gl/1CkT7 " 
A person who had the error posted about it on the official forums, and was immediately banned, presumably as it proves they were pirating the game. 
It's always possible you're getting this error by mistake. If you actually did buy the game and you're not lying about wasting $10, try contacting Garry Newman via Twitter or email, and good luck! If you pirated the game, too bad. You should go and buy it.
(Edit: fixed punctuation and sources)