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.
Note, _tbo9_cmeptb_4, SuperJLoew, Maner, knippnele, Gorn, omarelgamer3000XD, |TR| _SIR_NOOB_, abc303616, [W-G] Dan, Lenior, Fearless, karabear529, cooldawngamer, MigolitzXDD, T o T, gringbanderdam, _xav4234_, ScaryZ, The Glizzy Gladiator, juri_181, kelemenjozsoka, UncleSHREDDER, markodaniel365