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.
mossesx, re15revisited, Asyrummer, szabi1997, Heckelex, ☪.Space:Wolf, =GEN= VAMP, master chief, Korvyn, rollerbob00, Decin#Smile, Bud Spencer, PavelCZ, Yoyot64, proli2000, joel, nope, [SFL]Madde_20[GER], MisterCraft222, zozo890117, Elanis, l.maire c, FoxProject, MrJGASNIER09, Wolfman19682005, NuLL978, fluse1367 [Thofl-Server], Matrix_Gaimz, Picard, Mr. Panman