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.
ana25092001, root, ratoburguer, taoscatamarca, 룬빛싸랑, jasallcentrumcz, cokunemir, Erelio pelėda, ✪ Flamex ツ -iwnl-, DBioxt 21, vitor19laser, WESH ALORS, Mr Oliver Fox McFuckNugget, marco-antov, Mr Soarin, goncalo.lameira, aa8326138, sansarr, matroxz xDD, razavi.amirmasoud, leolove055, Bòizzz, david, munroedominic218, Matt [Photographer], Alarm, mariaojo69, Myksh Puyn, XD, 悟空之魂 勇氣神 戰神, 悟空之魂 勇氣神 戰神, iDarkonada, Jenny Taylia