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.
u.b.t e.c, pelusoide, Realtek, rex52528, annasmol44, leeysioson, exoticxdrill, jeffreyromero019, bergers03, L | OutlawParrot, Mr Gibbo, SlavSamurai, Samurai, dukenukempc886, IssaAlien, LittleAve7926, nelsoncoc64, Tekhnic, Postal Dude, Trinixity, eywgsg1223, , jessica.miranda1974, authoritarianammo, trollballin, tyeeeid, VREvangelist, LAG, TheZid, memes, Stephanie, Stephanie, Stephanie, Stephanie, Stephanie, francogarcilazo998, IzNoGoD, Nakey Is Bad, MattRTGB, Dr. Dendro, Dennis Stanistan, I LIKE PENIS, Camoll, PL Haze, PL Haze, Vegas, Emo, alpha20, Sannanore, danjchapman +872 others