Most likely what happened was that when you joined, the Steam Workshop Content you downloaded while joining got corrupted somehow. SteamApps will most likely be located in C:\Program Files (x86)\Steam or C:\Program Files\Steam, unless you installed Steam or Garry's Mod to a different drive. Follow these steps in order: In Garry's Mod, go to the Main Menu, open Options > Other and then change When a game server tries to download custom content to your computer: Allow all custom files from server After selecting that option, click OK Exit Garry's Mod and wait for it to close completely It will no longer show up as Running in Steam Make sure Steam is done downloading Garry's Mod - Workshop Content Click Downloads at the bottom of Steam to see its progress If it's In Queue, click the Up Arrow next to it to make Steam download it Start Garry's Mod and wait for it to mount Workshop Content The Workshop notification should no longer appear at the bottom Try rejoining the server If that doesn't work, try Validating Garry's Mod: Exit Garry's Mod and wait for it to close completely It will no longer show up as Running in Steam In Steam, with Garry's Mod highlighted, click the Gear icon on far right, which says "Manage" when you hover over it Click Properties... from the dropdown that appears Click the Local Files tab Click Verify Integrity of Game Files... Wait for Steam to finish validating Garry's Mod Once it's 100% complete, Close out of the Properties windows Start Garry's Mod and wait for it to mount Workshop Content The Workshop notification should no longer appear at the bottom Try rejoining the server (Note) Validating Garry's Mod will uninstall GModCEFCodecFix if you had it! Just re-run the utility afterward to re-install it. If that doesn't work, tell a Developer or Reply to this Thread for help.