Porting the map to Garry's Mod is very easy. In your game library, right click on TF2 and click on "Properties". Go to "Local Files" and click on "Browse Local Files". Click on "steamapps" in the navigation bar and click on "workshop". Then click on "content", then "440", then "696554453". Right click on the map's file and click on "Copy".

Then go back to your library, right click on Garry's Mod, and click on "Properties". Go to "Local Files" and click on "Browse Local Files". Then click on "garrysmod" and scroll down to "maps". Click on the "maps" folder, right click anywhere in the page and click on "Paste".

This should (mostly) bring in everything including the models, textures, and sounds. You might be missing some props, textures, and soundscapes, but they're unnoticeable unless you look more closely. Let me know if it doesn't work.