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.
Omegashooter, michal102008, PIESEŁOWE key-drop.com, Mafeotu, axelras382, ✪ NeCoO ✪, Bl4ze, com.antilles, so.long.georgia, erstra1, miguelmoyag, alejomar65.1, marialoveridge1978, 프7운3, -〖TR™〗༺J꙰O꙰K꙰E꙰R, Kotki123, Who İs Bey, ♚, <KEPLO>, nattapatdin14, Vince Lexus, fridgeit07, pumciayoutubePL keydrop.com, efkarrst, ExLy, caleb.shover08, GIUSEPPE, bryanlovescats, mojangdaihoang, ggxd039, asdfrt1477, bigwhite024, bartoszek353key-drop.com, erestontoypunto, Pro MobileYT, sasadef744, create, mefe.mefehan, Empathy, FrakYT, napiyonbeya727, lucianoruvidarte123, LUXD, oulhassimouhamadfaycal, ilgarKey-Drop.com, tomwis, Ninja_dec, yuro0217, mariasanmartinfernandez240, [SVIP]SG +111 others