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.
sylwester221, Xoneman, 20darnol, cozyy9662, czuhairdelrio, The_CanadianSanik_420, danieljade321, pedromorgan, 베텔, 496204347, Kuba123, wolfieczsk, ★★★, jesseconnolly9, SamantaRaioLaser, Serjikjkeee, HeadVIPShot, Hаглислович, evin17, supercreper111, shylohana, nomerxx123, earlernestovaldez, luiguigonzalz00, luke2012jones, xfbcb2015, Deadpool_031, valeriusmagnus225, disposable14, AntForce12, biohazardman, Helt Taffis, jeffthekiller96, jeffthekiller96, grantam1214, coyote1418coyote1418, Zazybos, AxePro16, truskiek1, the_rockey, Japalton123, Alex Mercer