Aprilon

Do NOT try and replace the default gameinfo.txt file found in the garrysmod root folder with the code found here as the game will still not work, the buttons in the game's main menu will be broken. Be patient and wait it out for Valve & Garry to roll out a proper fix.

The invalid code:

Spoiler: Show

Code:

"GameInfo"
{
   game   "Garry's Mod"
   title   ""
   title2   ""
   type    multiplayer_only

   "developer"       "TEAM GARRY"
   "developer_url"    "http://www.garrysmod.com/"
   "manual"              "http://www.garrysmod.com/wiki/"

   FileSystem
   {
      SteamAppId            218
      ToolsAppId            211
      
      SearchPaths
      {
         Game            |gameinfo_path|.
         Game            hl2
         Game                            |all_source_engine_paths|hl2
      }
   }
}

Problem to do with file permissions when Garry was submitting the update to Valve.

Patch on the way.

Jesus he couldnt orchestrate a piss-up in a brewery. Why don't Team Garry just throw in the towel and just let Valve develop the mod instead.

Hotfix:

Code:

"GameInfo"
{
   game   "Garry's Mod Beta"
   title   ""
   title2   ""
   type    multiplayer_only

   "developer"       "TEAM GARRY"
   "developer_url"    "http://www.garrysmod.com/"
   "manual"              "http://www.garrysmod.com/wiki/"

   FileSystem
   {
      SteamAppId            440 // 218
      ToolsAppId            211
      
      SearchPaths
      {
         Game            |gameinfo_path|.
         Game            hl2
         Game                            |all_source_engine_paths|hl2
      }
   }
}

Pulled from garrysmod gcf file

EDIT: It IS possible to still open map files in the game console
EDIT EDIT: However there will be a lot of errors and spawnmenu is borked

Problem has been fixed, you can resume playing now.