ph_ratrun_sewers.zip

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
          • ph_ratrun_sewers <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• ph_ratrun_sewers/
  • info.txt
  • maps/
    • ph_ratrun_sewers.bsp
  • materials/
    • maps/
      • ph_ratrun_sewers.vmt
      • ph_ratrun_sewers.vtf
  • settings/
    • ph_ratrun_sewers.txt

YisusWars, Kemphler, Dark_Obscurity_Server, Linkus, boblord00, The_king_core, Kaan_2002_yilmaz, matrix109, ASzaz ^.^, hakimadrayf123, Spicy Boy, ShaeOverRated, HowlingFear, freezingphoenix79, dandan, nik-nekich126, Demonn, lyannerz, Xeyar, 176cm 50kg 18cm, Samtherock, You, Ash, Manoz, Acnologia, TealAntlers, danendrasaskara, ✪ Nemi, Mumin, BLANK-IDxx, h3artl3ss362, AminTheMar, Fictional Wanderer, Liban, FishTahko, rockxx317, sonic, shefocrz, walaa_alhalabi, modadas12, Cat, HartM!, Dwayt, Business Fish, sonorc, OSITO (TED), Late Stage Dementia Patient, Flash1208, DeluX, general18 +1564 others