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:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
kneesockscanty, gryZor4000, FireFight, redhamatrook, 小白, Dross Rotzank (SDLG) ( GDLG ), littlemonsta, SlendeRAMK, G-MON, ☯Marisa Kirisame☯, WAFFLE HAUS, naneth kyuren, mushsky, chlrud951, 「§NG」| Cookiez, Henrik Ibsen, GTS, TheRam, Wspchn, Davide Scarlet, jokoki64, Miia's S.W.A.T. Team, >>Great White<<, >>Great White<<, trigger_hurt, athallaf, Mexican Man, ://neo.psych, ://neo.psych, don_sulten, ⑨Shendow, len_218, 65 Wat Compromised, Sebekpolⱥk51, hel2mes01, Zalgycake