Half life 2 validating files
Tools and right-click on the "Source SDK Base 2006" and set it to "Never Update" 2) Uninstall the 2006 SDK and shut down Steam.
3) Extract the old 2006 SDK GCF files into /steamapps and restart Steam. I retained: base source engine 2source source source sdk source Those all together are around 2.9Gb (RAR'd 1.2Gb) and nobody would seem to be very forward with just linking an archive of the GCF files, so the only way to remedy this is to find someone who has the 2006 SDK who has not allowed it to update to Steampipe and has all of the Source SDK GCF files then link you to the archive so you can extract them to your /steamapps folder.
So this means, All along YOU were using the wrong engine You actually use Source MP for gmod 13.
Click Here for full instructions in the Server Administration Forum.
If you already know how to set up a vbscript you can skip this step. Dedicated Server  Name=Eternal Silence  Name=Eternal-Silence Dedicated Server  Name=Dystopia  Name=Dystopia Dedicated Server  Name=Insurgency  Name=Insurgency Dedicated Server NOTE: if your operating system is 32 bit then make sure that you write just Program Files instead of Program Files (x86) the vbyesno portion will give the popup box the yes and no buttons allowing other sections of the script to use the result.
Open up Notepad and save as "[your game here] fix.vbs" make sure that the file type is set to All Files. I will use Portal 2 as an example in this script because that is the game that i originally wrote it for but it should work with any other steam games that get this error. now we will write a section of script that will launch the game if yes is selected.
There has to be a reason though as to why the converted SDK is causing this error because it is supposed to be, after all, the same SDK as always.
I'm sure more information or a permanent fix will be worked out soon.