So, i’ve seen pretty much people have this problem. I found the fix for it.
Mostly it always says that error when you try to join a multiplayer server.
First of all, exit/sign off from your Steam.
Then go to your Steam main folder (Where Steam is installed) Deafult is - C:\Program Files\Steam
Now if you’re there, search for a file called “ClientRegistry.blob”
Remove that file, now if you start up Steam again, it should be doing like updates and stuff.
After thats done you should be able to play multiplayer in GMod again. I hope i helped