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.
Jumpy, qqu38888, БОЙКО, unclebody, Smile, NOVICE_WANG, Pumper, Glitter, CLOM, Angla, TOO-BEE!!, Magic Shmagic, crester, labrats45, PigeonHawk, Someone, Nic' *Logitech, v.pfahl, THE COMBINE245, GOD-VAPORE, Direxe, aidanspalding1, Wishmaster, ·٠•●ProF!K●•٠·˙, SHOOTABIRDIE™, [FG] Shark_vil, Either, [HLWS]Fr4nkM - I'm Outie, [LFG] SaintLuther, CSGO500 TLC 2.0 | Ever, Sirdog, ONX_Tronpa, ONX_Tronpa, Ricky, Nuker, [LulzP]Denz, Redline, Natolikin+, II_PYRO_II, , yao6killer7, Keshwoof, |RG| BadCompany, nah