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.
teryoxin_2000, Coccone, Coccone, gandalf, Drake3423, josh.g469, boopjack, Angry Guardian, adamhreben, lcpsabsalonf, Scriptman, Metrocop, turkufiliz, Lord TL [Hun] ☭, Loki, richaz, proli2000, Steventechno, olivierchausse, vinivallim, AMERICA!, Magic_apple_tree, gampnick352, steven, Matrix_Gaimz, bonjour_twa, GamerNicolasPR, Har Mar Superstar, artist1375