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.
3499387031, thebaconnoobn5, ultima_sword, rnhb, combine_stalker1, celilahmet1453, tayloralthoff0, doraestetica12, navalcoy, rohanpark871, kutetop000, aundra831, KRiSTiAN, xbreshanaj6, igotbored112, BabyJaws21, Mr. DusMees, edugamer323, darius222, Fakeguy18, TerrorBite, mimod17, LuluTheMotherLover, too many edgy outdated memes, universe, nicitaofficial5, thekidofgaming, gamessamuel95, Ayranmen, MeeM, zSkyBlooD, Queen funtimefoxy, mossesx, deecrolla, hunter____zombie__killer_205, Микаэлян, DropLocK, unnameduntitled, taner08051977, Eternal Jaxton, XxHwaTheStrollerxX, HotHead_TH, PaulStefano, akanson1, DOOMGUY, Q0GAL-7QJLH-DI08I, 05453606324m, maks.reshitko.04, =G.E.W.P= Pootis Yoshi, sausage friend +277 others