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:

• steamapps
  • common
    • garrysmod
      • garrysmod
        • addons
          • smartsnap <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• smartsnap/
  • data/
    • smartsnap_default_offsets.txt
  • info.txt
  • lua/
    • autorun/
      • client.lua
      • menu.lua

longshanks_mcgee, FOXIGG, nicecrafter0507, bernaljohnny3, Dominic W, BLINDÃO, [MFT] Xtazii63, Russiaman, Bariore, marsh, ☮RamzeS⛓[BRG], ☮RamzeS⛓[BRG], edgefighter2017, Yordanovvv, Marco22, antoniusevans93, Conar, Cayde, DIDIX, Grey Silhouette, mikosikus123, Bezimienny_33, kubikmachac, noodlemop, noodlemop, xXSoundBlastXx, [IFN]NickF[TRR], veran.matosin, FUCKU (╯°□°)╯︵ ┻, nicole.velic78, kkowalczyk124, DeWalt {GsG}, SeekNDstroy, RechteSchamlippe, Salty Cracka, iPflerDotNet, Kevin006, walid-gaming5, sampistola, <hsv> Melvin Tyson </hsv>, WinFORTUNEraulandrei123, peeyshkumar.25oct, Void Madness, eitanfrancisco, lauty1905, Binbin, QUBECLAN, GewooE, Dragon4ik, jord9six +24 others