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.
bankpe20, hauntified, miguelsuarezobando, x.pegasus.x06, Doommehren, GG-Gabriel YT, mj9309054, colehines2, vidalcormary, Ghost76, kirby el mejor amigo d cafecito, MeltingRUSH, qqqtrs, mustafa97.alacam89, szabomark271, giedrejurksiene9, hahahaiffinity, tayer45_, calebbond2007, Luizdoa857, Luizdoa857, elbo1152, VicenteK, Diadia Vasia-Vasilij, olioli2061, kristupascarn, matteo.v dealskins.com, ☠titanz91x☠, kk, yoryimendez02, lampika3, job_finder, dwayne-theeten, 03kj20f03232, rahimyi147, KRiSTiAN, KRiSTiAN, KRiSTiAN, KRiSTiAN, christian.barber42, xbreshanaj6, UFSans, armoody10, mr.fogus.2547, battleman, seancondino05, seancondino05, burritotos801, zaahirjoseph, alfunk888+charlie +49 others