Hammer missing textures

Isn't this kinda obvious?
You need to have those custom textures locally to view them in hammer.

I hope you have permission to edit this map. Don't release a modified version without permission of the original author.

PakRat

The most convenient way to add the map's materials to Hammer is to open the map in GCFscape, and drag out the ZIP file to tf/custom in your Steam Team Fortress 2 directory. Extract it. The result should be a folder in tf/custom with the same name as the map, and inside, folders like materials and maps. You will need to restart Hammer for it to load the new materials.

When you have finished looking at the map in Hammer, create a new folder in tf/custom and move the map's folder inside it. Alternatively, just delete the map's folder. This will stop Hammer from loading the materials, so you won't accidentally use them in other maps.

Or you could just ask the decompiler to unpack the packfile for you.