This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:

• steamapps
  • common
    • garrysmod
      • garrysmod
        • addons
          • RP_Downtown_V2_UM_Sources <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• RP_Downtown_V2_UM_Sources/
  • materials/
    • dttrains/
      • dttransit1.vtf
      • dttotrains.vtf
      • dttotrains.vmt
      • dttransit1.vmt
    • theprotextures/
      • blendgrassgravel002a_gmfix.vmt
      • weapongui.vmt
      • dtv2admuncp.vtf
      • dtv2admuncp.vmt
      • gunshopsign.vmt
      • RP_Downtown_V2_UM_SignTexture1A.vmt
      • RP_Downtown_V2_UM_SignTexture1A.vtf
      • weapongui.vtf
      • gunshopsign.vtf
  • RP_Downtown_V2_UM_SignTexture1A.png
  • RP_Downtown_V2_UM.vmf
  • scripts/
    • soundscapes_RP_Downtown_V2_UM.txt
  • HEY_ASSHOLE_YOU_BETTER_READ_THIS.txt

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