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.
kiselev-vadim-05, nedeljko52, keamicero, adrianshepard, NobelPL, Ogaiht, adrian.dd56, thegamedude221, cobcarr1, kyshon25, hangamer 69, hangamer 69, ♫ Wub Master J ♫, barselona19762005, Iwonym, dexter, Korneliusz, CommanderAvtomatMakarov, sukoisu47, sukoisu47, haydon546, Legoben2, k.a.c.t.u.s, thewildbirdmedic, thewildbirdmedic, =PP= 419