i guess the new update that broke gmod will be fixed soon.

I joined facepunch group on steam and started chatting and this guy “Germz” tells me:Garry has finished making the update. He has sent this update to Valve already. He guesses it will take 4-5 hours for the update to be uploaded.

This was at 17:40


Garry tweeted that it was fixed 6 hours ago.
Also http://www.facepunch.com/threads/1117242-Another-engine-update-broke-gmod-won-t-start-up-*KEEP-DISCUSSION-IN-THIS-THREAD*/page8