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

• glicence/
  • addon.txt
  • lua/
    • autorun/
      • glicence_init.lua
    • entities/
      • glicence_npc/
        • cl_init.lua
        • init.lua
        • shared.lua
    • glicence/
      • config/
        • config.lua
        • lang.lua
      • core/
        • cl_main.lua
        • sv_main.lua

canadianninja4, EnvyzZ YT, SamLedze, (свобода) смерть, Michel Tenzi, Mika Délirium, UrPsychoExGf, leproducteur.gaming, BenG, YSL, mirrda06, STEALTHY, ◥◣◢◤ Boss◥◣◢◤, nicollecadiz, Delgo Sama, petukhovdmithry2018, Redboomer123, Cheddar Cheese Whizz, ErykGamez, FoxYTGaming, darth Killer, terripri819, Bulli, UziBoy146, kristupascarn, etheth19, damianYT1 | SixGiercownia, twatboi777, sloth., lobbyps3chezvins, Pygargue du Zimbabwe, Ellis2K, {OG}LittleOlivia, swaller4222, mr.pixelsyt, Greg69100, Spiky, Pop Korn, RoyDaKing, emilienaubineau2002, gonzalitomartincho, maikypotter59, Estebouboun, oefe3034, Kara-ZO-REL, Deadlox, boboplay08, TouristicFiber, MrBoss, tom_pie19 +70 others