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

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

• steamapps
  • common
    • garrysmod
      • garrysmod
        • saves
          • gm_flatgrass0000.jpg
          • gm_flatgrass0002.jpg
          • gm_flatgrass0003.jpg
          • half-life-000.sav
          • half-life-000.tga
          • gm_flatgrass.hl1
          • gm_flatgrass.hl2
          • gm_flatgrass.hl3
          • readme.txt

• gm_flatgrass0000.jpg
• gm_flatgrass0002.jpg
• gm_flatgrass0003.jpg
• half-life-000.sav
• half-life-000.tga
• gm_flatgrass.hl1
• gm_flatgrass.hl2
• gm_flatgrass.hl3
• readme.txt

zugdidihostel, Another generic name, Dewtail, melentev.v, coleharmsen, renzo80cr78, inalefe3, kranister000, Whysky, АРБУЗ manncotrade, an16krul, edugamer323, MIster_sasniy_PIsos, abi.sofiane, andrius.andriuxxs, CanalLucas2006, guy2246245, xXTomeroXx, ragiell.jengjengjeng, juanfranrecer, lukaszgba, mistergamin1, gabor_cristian013, #o1YuSuF, e.treviss, vladkurpyn56, pepupi, scriptluablox, Bax, LittleNoobie, zeroorange39955, michaelangelodagaas, RadixGameplays, Beliar, matt58basket, mgeek97, apWE2007, naskata2003, yofojazer, The Maddog, miknik21102006, MRBASS, w21drakec, paola., remember17490, payton.foggs, csgo.net 2754, florin21pruna, Tron, Brambasiel11 +479 others