Extract the zip file folder.
Place all the folders in the garrysmod folder. 
(This optional for little extras like map thumbnails)

You can just load the map .bsp without the other folders if you wish with no problems.

E.G.
C:\Program Files\Steam\SteamApps\Steam-User-Name-Here\garrysmod\garrysmod

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Note to server admins with Fast Download Servers. 

Here's a direct link to the Fast Download File (.BZ2) 40MB
http://gtamike.tsgk.com/my_maps/hl2mp/maps/ttt_nightmare_church_b0.bsp.bz2

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Map Made By gtamike_TSGK 
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Map Update List (B0)

- **Updates**
- File Size Compressed/Optimized. (40MB Fast Download)
- Traitor weapon spawns removed.
- Small Bug Fixes and Tweaking.
- Some texture changes and new grass model sprites.
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Enjoy and leave comments to show support. :) 