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.

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: [url=http://www.garrysmod.org/downloads/?a=view&id=118348]http://www.garrysmod.org/downloads/?a=view&id=118348[/url] 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: [url=http://www.4shared.com/get/YG0zSf6s/CSS.html]http://www.4shared.com/get/YG0zSf6s/CSS.html[/url] • HL2:E1: [url=http://www.4shared.com/rar/IHOJw8IX/episode_1_v2.html]http://www.4shared.com/rar/IHOJw8IX/episode_1_v2.html[/url] • HL2:E2: [url=http://www.4shared.com/file/Pgolye29/ep2.html]http://www.4shared.com/file/Pgolye29/ep2.html[/url] I have -not- tested these; but 'CSS' has been confirmed genuine. Remember to: [color=red][u][b]USE WITH ABSOLUTE CAUTION.[/b][/u][/color]