ok it is easy!
1: just rightclick the games that wont mount on gmod
2: go to properties and click the betas tab
3: opt in for the steampipe beta and close! no need to worry about the beta access code field!
4: launch the game you activated the beta on to start the update process!
5: enjoy gmod with the games mounted
you need to do it for all games that wont mount.
SteamSkin: Metro Skin
i was not sure where i could place this thread so i hope its ok i could place it here