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.
julia.anjos.correa, alissonps101, tjxd3652, Domino's Pizza, stereofoxmuviz, TKCOWO, thailandtin2548, rasniceric, Le ArkHozZ :3, thebamgaming9, muhamed6969, muhamed6969, oteplives, DarkBenjamin19, Sharpy-Owner.ZombieLand, oghhmed, uneducatedmilennials, cerpektyle5.0, Zeus221, Articlegamer, Stupid Person, zskninoh, Ben 10, mennY, INST4K1LL, lapze45, L4ZUR3, Tsumas, Exumer, Hollandkiller, luke gallgher, charles.r.francois, red_tyrant, red_tyrant, lucabog06, epic_is_my_game (Master ❂ Ninja), ikaroskiller, Chernobushka_repoter, Chernobushka_repoter, FAITO456, You are dead, not big surprise., wayneaking95, heromcpe, Life, selskiycat45, Leo, Otaku1Up, lostnails, cibershot2016, XxPOOTIS HEAVY101xX +163 others