This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod folder.

You should end up with a directory structure similar to below:

• steamapps
  • common
    • garrysmod
      • garrysmod
        • lua
          • autorun
            • EasyPrecision/
              • info.txt
              • lua/
                • weapons/
                  • gmod_tool/
                    • stools/
                      • easy_precision.lua
            • SmartSnap/
              • data/
                • smartsnap_offsets_default.txt
              • info.txt
              • lua/
                • autorun/
                  • client.lua

• EasyPrecision/
  • info.txt
  • lua/
    • weapons/
      • gmod_tool/
        • stools/
          • easy_precision.lua
• SmartSnap/
  • data/
    • smartsnap_offsets_default.txt
  • info.txt
  • lua/
    • autorun/
      • client.lua

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