[scruff_tech]_pet_headcrab_v2.

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
          • Expression2 <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• Expression2/
  • [Scruff Tech] Pet Headcrab.txt

Coffy, Klops, Salami Sword, carlosriego1999, ◕Δ◕ StarWolf ◕Δ◕, [RG] Pedro, Le_vibe, Le_vibe, WildToastyMop, 彼岸花落, LAZER™, LeTueurBronieDu66, DarkShadowDmitri, ╰☆╮ sarah ╰☆╮, [KdH] Terax, DemonLordDan, [email protected], [email protected], [email protected], Artes