One key thing to remember when using Advanced Material, is that you have all our content installed by using '!content' in the chat when on the server. This makes sure all textures are installed in your Gmod files, so it allows you to access them for the Advanced Materials Tool.
Under the Browse Materials section in the 'Q' menu;
Browse Materials/Addons/3/xranger/(expand file) - [Walls, Floors]
Browse Materials/Addons/3/typhon/rpberlin/propper/ - [Walls, Floors]
Browse Materials/Addons/3/xranger/(click on file) - [Posters, Banners, Propaganda]
These 3 file paths are basically all you need, they are my go-to and provide Prometheus Textures in the current map, as well as older Typhon textures which can be incorporated into builds.