Yep. A known fix has been found by yours truely, me.
You want to do a few things. First you should know that CS:S doesn't have the SteamPipe beta 'Opt In' - which leads us to believe that HL2 EP1 / EP2 are what is important, which I've discovered is true.
Right click both Episode 1 and Episode 2, then, under a new tab on the far right you'll see where you can Opt into a beta. Choose the selection "SteamPipe". Try to run both Episode 1 and Episode 2. They should automatically prompt the format conversion, after which you simply run the game, then exit and do the same for the other one. And check to see that it's mounted in GMOD.
I'm not sure if this was necessary, but I checked to ensure that my CS:S was fine, as in to make sure that it also converts - if it doesn't prompt you, you have to manually uninstall it, remove the folder in the Directory "\Steam\steamapps\common", and then proceed to re-install CS:S. You should now have the message saying it is converting the file format, etc.
I just loaded Garbage in single player and there are ZERO errors now, seems to have fixed this issue for me.
/sigh of content and a dignified strut