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.
KraqparK, MurdyGamer, torkel.nykvist, SirPanda, adrianmagureanu100, Beta158, enzocrackcomogoku, >:v, Lietnaunt, bmwincorporative, KG-6 Sleipnir, jennie.antczak85, PanzerBlitz, Winter_Soldier507, tssthesplitscreen, Raphael, CLANK-O THE AUTOMATON 1.0, Moose, Holdgaard, sjaak31367, samuel.jesusfreak, spongbobf19, HeadSharper, Razek, Many Poofs, moustafawolfe1, JoeyWorldTour, fenixwarrior1990, Cabada783, barusu5139, hcolby0314, dkellarNMHS, Ferbal, ahunobat, Shugou, Chase1.9, josemaria_zuba00, dminivan, Nik-tuzow, Nik-tuzow, MaoTseTung, anaidalillo, rpg16, -Javi-, Nathan Drake, BuddySkai, Gush, R1ch, eggowaffles700, Hero989 +824 others