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.
Khoda, misgames69, The Gifting Ranger, thailandtin2548, Icaat, Icaat, Vesuvius, saana, rodrigovelezdiaz, »ƒŗεεđσm«™ Λŗςhdενίl ✯☭, Private Eye, Itjaritjari, overki11, talhaqadeer408, Krobus, Krobus, DESUKON, adampro2001, UncleSHREDDER, VisorMachine™, damian.nowis, 3D_Girls_4_ever, Classic, ☭Slaier☭, Wolfman19682005, Wolfman19682005, do_i_ob, QuieT