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.
dasilva charles, Vincent YT, Phoeniks12214, KillSwitch|AmericanRP, GoldenAppsYT, Floppy Disk, Zopalo, Mr. Nyxx, IG|Kelytro, Robin King HD, Bishop, fromentvalerie, | RPgames, NigaVideos, PoorTuna, HotDog, england is my city, 5Eyez19132, realrbn, SEGA, apl2, TheRedSpy, TRUE MIND, [SIH]Lemar Bonquiqui, Anka, MrFlabbyFlaps - Mesagaming.net, !ImNew!, mike.britt09, Zercky, топаз с украины, code_gs, code_gs