That error message basically means that the server has crashed. I couldn't reproduce the problem with the current release of Garry's Mod, but I was able to get it to crash with the development branch.
Are you actually running the "dev" beta? You can verify this by right-clicking (or Ctrl+Click) on Garry's Mod in your games list in the Steam client and selecting Properties. Then click the BETAS tab and look at the drop down list. See the image below.
If you are in fact, running the "dev" version, I've updated the first post with a
new download for GMod that should work with both the current release and the development version. If that doesn't work for you, then there must be a different problem and we'll have to try something else.
(Regarding SteamCMD, appid 4020 only has files for Windows and Linux unfortunately. For OS X, you'd have to use appid 4000 and log into a Steam account that owns the game rather than logging in anonymously.)