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.
lampalonpro, WindSuko, Lil_Eze_08, annoyingmikamail2, Napp, MrCheval, olivier.gerard20, Space_Mex, Dark29086903, yelitzacarolina, yelitzacarolina, yelitzacarolina, Olegysha, Eleven, A. Mike, QUES, Maximous, Лютый_zxc, impto, karpinskipatryk399, sava4ka, mmm onyx, Maiky, provgamer50, МШК ФРЕДЕ, Bomba Jump, H11pper天今, comedysauce903, celyxl, That Box, adrimooima1970, я не B1t, andrejkip666, Fijdi_X, jacobaukan, opisynthetique, Koro Reyes, Toine, Saman.exe, SoLovey, gameslevatare, RayZone, Ленин, NineBoot, xausmaster449, S1i_z, S1i_z, deidara.sasuke.martin, kaktus, CoLovey [RU] +3469 others