Are you sure you have a legal copy of Garrysmod from Steam? If you do, try making sure the Steam folder is not Read-Only. To do this, right click the Steam folder in “C:/Program Files/”, tick the box “Read Only”, and make sure it’s recursive (applies to all subfolders and files). After windows is done applying the file attributes, repeat the process, but untick the box marked “Read Only”.
If this doesn’t fix it, navigate to “C:\Program Files\Steam\steamapps\witty-username-here\garrysmod\garrysmod”, and move gameinfo.txt to your desktop so Steam can try to regenerate the file.
If this still isn’t working, try running Steam as an administrator (If you are running Vista).
My last suggestion is to verify Game Cache Files.