You're missing partitians of the 'required' content - such as:
• Hl2:E1 & E2 [Optional]
• It -may- be a 'FastDL' error; but I honestly do doubt this.
• Re-mount -all- content that is available for 'Garrys Mod', do this by: Opening 'Garrys Mod' -> Bottom Right DPanel -> Click -> Mount. You will have to restart the Application for it to take effect.
If all fails, then it could be a FastDL or Cache problem. And use this link:
http://www.garrysmod.org/downloads/?a=view&id=118348
Also: Keeping in mind with what Calver previously stated. I'd suggest the following if all above do not work:
Place 'CS:S' files from 'steam/steamapps/name/counterstrike source/<copy the 'models & materials' folder and extract to 'steam/steamapps/name/garrysmod/garrysmod'
^: If you're using any custom materials, then that's your fault.
And/or [Recommended] download this file:
• CSS:
http://www.4shared.com/get/YG0zSf6s/CSS.html
• HL2:E1:
http://www.4shared.com/rar/IHOJw8IX/episode_1_v2.html
• HL2:E2:
http://www.4shared.com/file/Pgolye29/ep2.html
I have -not- tested these; but 'CSS' has been confirmed genuine.
Remember to:
USE WITH ABSOLUTE CAUTION.