This download looks like a map. You need to place it in your maps folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
Umbra, hs_doubbing, El morty mas morty, Beyond Nature, slave_st_ford, 277141, kelemenjozsoka, RED Floyd /(Weber)\, serkan_guler18, trapeza.com, karman02, jose-alejandrox, Master Anyon, btt.7698, bautista13angelica, ChalkSauce, errollees, fenixx234, Hösjolm_Alexänder, Soii, Benjamin03112, gangster elmo