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.
[CrM] Zelitrem, [SRP] T4TSUK1_D&SP/\Y, Verderex, three burgers deep, Spartan6914, [CRP] Luivi, FalcoModz, Sexy Frog, Information Highway Police 23, Jonco, kronik, CaLeb, Angrypygmy, Angrypygmy, Mr. Conway, Mr. Conway, 11 10 15, smt