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.
maistro_78, NCRP YouTrynaBox, collinsart12, [IFN] Hayzee [HA], luca_cnl, [GL] Divide, ♛Frozzen., bogdan.sirbu5678, stalin_odinokiy, <hsv>ThiccSlicky</hsv>, sickboy, World, Purple like, ⁿ, Gottman, ruzgararcaner, xex, TheAmazingGod, Jessica, urlife, ⋘ֆ₳ห∑₭⋙, Spanch Bob, asuna, [CSPG Pfc.] -Spooky Courier 6-, Lil Admin, Kisame, Dovzzik.dll, toniusx, Spatnaz, , Falioor, slimymeekrabbit75, АLXRP ниманик, heet♥, ScreaM, Wunker☿, Nekyto, Nekyto, idk, КОФЕЙН, Officer Big Mac, deividasqwer, Baron FlameRP, D3CK3Rx FRIENDLY BOYZ, Mariosprogamer, [PAC] Funstab, KingKitty | SuperiorServers.co, Bobby Car, Bobby Car, Err0rS [SRP] +549 others