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.
re15revisited, tothandraas03, harrymurray2004, edgarsveinbergs22, clinelori63, Afengo #DONATECARDSPLS, unknown error 11, Mera.meN, omar.ashraf2000, kemalst34, Jurisdiction, fadi992009, Freddy43, patriot_trox1cl3, Karadoks, GamingCap_724, boblealeonard