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.
petukhovdmithry2018, zekealoo, v.solcik, toxicbboyz, Brambasiel11, Nasty, kamilpolak04, Gamleth, Zeus221, eliseev-valyusha, MisterAlpha, re15revisited, El negro confundido :V [Shei], kubik.horak, teryoxin_2000, Sans, BountyHunter29, Tasker, Wabba_jack, cyclone0011, rileyjay_5, rileyjay_5, hoodsaiyan, ryaaaaaaaaan, Ｋａｙｘｙ, NEXUS, alejandronarvaez.ang, ⏚, =[o.W.n]=YuriThe76Soldier, Raccoon&Co, coleman, raihan533, •۞۩ MICHOU ۩۞•, Demon, good gamers, mihajloweed, adrianraicu2015, Ultrastar 1, clonemarauder, fps-creator-glo.ucoz.ru, JAN.HYMR, [UG] Noffe, Jack Baker, calimino409, xTheMicha, Ɖ⌂<·⌠ℤ⌡, Ɖ⌂<·⌠ℤ⌡, lukas.lh, Chaoslord456, Novan +40 others