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.
jwvk28, andrewjgroen, sabinarohani66, leviy_twink, Golem_Commander, samuraidragon17, alanmonsterskull, Rezengrad, leonardon789, Romanich, CarringtonTheObscureGamer, =V=, Wendell Carlos, krazyxsnipez, benadeandries, Dimitris Panag, super.ultrapoint, James Fuccon, MrMooseYT, neccos, jonas.rumpa58, Deströyer666, Deströyer666, Cheeki-Breeki, J_Kreir11 (Dark_Ghoul), loree13m, NeroBG CSGOFAST.COM, Ry13y04, Ry13y04, mortalweny, tom, ririchoice, -0-0-, hillmeu, THE BENEFITS DON'T STOP THERE, Squeeze, scar.rab61, apac_420, (YouTube)natasha, (YouTube)natasha, xXFantasticPandaXx, ArkAJBoy01, bautista13angelica, Kenny, bael, drunkdekushow, neon dawn [BS], Celtix21 [40K-G], panos6532, Grj1315 +40 others