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.
Karma, connor, (YouTube)Наташа Косюк, Sacapetroleo, berkay_5530, Xenosiris, marcumaftei, owneil., [TWCGS] Nopity, tothandraas03, victor_9212, Heckelex, C.P.GREGOR., Alarm, Leander1515, ⎛⎝⊳ⒸⒶⓁⒺⒷ⊲⎠⎞, bertisis, DjCL0n-E, sn1p3rdu08, Binge Eating, GroSSoMineTTo_Mappeur, Savisaar The Great, Jafar may need glasses, Bubsy, RonnyRoss