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.
PIMEQ, ELDragonne, Ronnie Paints, mdoobs, spartan119990, Not Alex, pataprevratil, charles.field2004, Alarm, Vizier BeatCrazed, Delofon, alex_1313, stonelords12, MadDog, killeralex80, Wolfman19682005, Cpone, senpai™