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.
Bread, Балтика 9, fakesoundsmith, LeaveToJoin, .ƊĹ, Hawaiky, Black.dx, Zaamurets, Zaamurets, tankopenyt, †α†απ, m_athi_s_24, mr fluffy blue, liang20070309, Mr Giwol Lumberjack, ItsJustMiaouss, berne.gregory [RockFordCityRP], Sir Scruffly, MasterOf88Keys, BlackZARG, ☄, ZombieKill3r, Mendhi1616, lolteam/Craig-Davidson, Deano, Mr. Dispenser, [LOS] craisyman000, Adrian_el_mapache0, Chungus Rush B, maks, UssopSniperKing, ZGFueDkx, -Ng- paydaymydude, [GN] Charlie, TheUnturnedOne, Tonny Merguezz, Tragiiks_, Sebilein001, ♚ Axios ♚, Fléodon ✅, =WL= LineyMuffin, Ancorical HawkServers.co, RobinDerection, Patrik Lacarpe, FireCracker is Free!, CracklyEmpress, UnicornSwe, TraphinxX, CraziozFR, rstclm +83 others