This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
Banana Network, lulubernut04, plianes766, SH // MleKo, Thomas Patoche., PEXIPEE, Jemelox, Tupac Shakur, ∆ᖇøﬡ∆ Felix, Babyhamsta, [L²:RP] Arch.B, Rire, Rire, Smor, Sr. Alface, David, Jose Hernandez, [FL:RP] |TBB| XxMrBubbles18xX, Pukki, Wepnet, Nick78111, yung dab, Dusty, GarlicBreadPro, [L²:M] Cameron, [B.M.T.] Soong [TH], lonely19682005, Froge, apple, Sir Aussieman, Collin Craig GMOD2 CONFIRMED OMG, Skeleton, JustMyles, | AG | JohannesNeo, Nathan77, azzen, james, Ermen07