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.
b_blutklinge, SleepLe55, jhon smith, Destiny, Gurev, Better Dead Then Red Nigga, [๖ۣۜRőYaL] Toka, zadquielortega, DanteXYl, , RAKOUNNA MATATA, $assy, Redstarwolf, [ACG] Arena Creator, MpL, I'm Commiting Cancer, I'm Commiting Cancer, Sir Kitty, lukefulleylove4 [TRAPNETWORKS], SMk, [VR] ℳeh_ℓØω™, da7mon12, Le Gouverneur, [C.C]azi17001☆, Spugless, MRRMONKEY, Frisk, Ananas, Arytoms, Neptune, [GL] Left Shark, [Fondateur T.W.DRP] Punkiz, boyjannik[German], Balthazar, MaLeTut, MaLeTut, Xemith, RampageKing11 [GFX], r4mpage, Izzabo, 1spizi77, Douglas, Ban[G]bust, GitGoosy, D3lta, [NWA] Wabbit, DeathAngel, Kingmaker, Trouble, [FondaEvolife] Tieno +263 others