As I have been receiving a number of people asking how to get the texture I have created this discussion explaining it.

This map requires counter strike source textures which can be obtained by purchasing and installing a copy of counter strike source, many workshop maps require these textures so it will be worth investing in the game.

Once you have bought and installed the game you then need to go to gmod and mount the content to the game from the menu on the main menu you can then play this map without any errors.

Alternatively you can download and install the textures from a third party however these are not guaranteed to work correctly below is a tutorial that i have found effective in doing this.

Below is a video that i found to be the best working way of getting these textures for free.

https://www.youtube.com/watch?v=yaL7gISscEY

I don't take any responsibility for the files from this youtube video though as i'm not the poster so download at your own risk.