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.
eliseev-valyusha, fett.j, okris.fernos, lbronner, тsсм.JohnSheppard, freddyfazbear360, mathew27, mathew27, nick2200, SPACE, dvach, Gaming Universe, victor_9212, patrikskola11, ZAMOSC102, soundman2000, Heckelex, Little~General, debourom, Loki, zippitey2, Shadow, proli2000, jonathanagopiandu77, Marty #ThugLife, |Tғ| El Oso Mieloso de Lucy, ИЛАРИОН-ГЪЯРЯК, Vinny Vincenzo [FR], Iced Snake, Magic_apple_tree, yanipku, [BA] Brucektrain (DEAF NO SOUND), TearsOfDream, The Doctor, Picklebond