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.
marcjosefrabina, victorherrera, Murky, antoine2.hascoet, аБИБАс, Rey6Ghost, Rey6Ghost, virtualfrog1000, kenistudioind, bhercegovina679, Націоналіст, gabrielmaglov20, 240 Km/h:PEEK İsmigül, Dew Riffer, pany2384, burakdurna2733, reyen999, aynurdogan476, aynurdogan476, Varga_Dominik, PTX_9017, WhatDeDoq, prekpalaknikola, Madzikalll, Key-Drop.pl, corachita2018, moni_950, volkogon212, zsozsika8000, VDominik HUN, LorYT, [LyA] Quebs10, James Ego, jhevejoshualibatique, Long Nange, EL INQUISITRON, Alex_SEMAR, mathew.ravelojaona, 神秘人, Balkaptan, soypro.ribo, Suddenly Astronauts, Patricia, KERRY, elgeosprods, elgeosprods, elgeosprods, Point Man, [2P-01] Tyler John