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.
Wolfy_, Ackua, .#Zipcore, Jack, Rire, sandiegoklassik, KeZzY*, =PP= 419, ⑨Shendow, Mr. Brightside, Keshwoof, Keshwoof, Yew, [GG]Tense!
Submitted by garrysmod.org