This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod 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.
[MS] Wolfy_, Waykko, Sorch, [W-G]KeithCrackers, Redsparten321, Console, hdghfthffh, [RG]Nicry1, YouyouLOL, BASH, SilentKiwi, Potato, Nuclear Rainbow, ★Gandalf★, 変態お爺さん | h-manga.moe, <<====-2Q-====>>, [BMRP:FF][≡FCS/FCT≡]Jo[FR], Blackbird, IPv4, GoldenRevolver, PCS | PhoenixShay, PCS | PhoenixShay, Rich Co-OP, Fur3uR, happy"_"bunny"_"carrot, Zaku [Black], [CG:A] Tom, [CG:A] Tom, Angry Engineer, Johny Camel <3 Major and kaN, Mr.Wolfyo, [P-G]Maddy WolvenClaw, [P-G]Maddy WolvenClaw, [P-G]Maddy WolvenClaw, Solar_Dust, Solar_Dust, LuckyLethal, xXRaDsKiXx, [GRPH-SA] Clanof20, Galtrox, Galtrox, MrGeekabit, MrGeekabit, Senateman, Senateman, jacob.wakelin, jacob.wakelin, [AG-CO] ▼DragonRocker▼, Sirlionfur, Sirlionfur +7 others