Garry's Mod error on attempted startup.

Hey guys, it’s me again, and I found something when starting up Garry’s Mod today. Upon pressing play, I get this error:

MountAppFilesystem() failed:
SteamMountAppFilesystem(4000,0,0x1a04fd24) failed with error 1: No default filesystems for this app.

Please, if anybody could help, I would really appreciate it. I have googled it to no avail, for all the fixes do not work.

There are similar things happening to other source games, none will start up.