The most secure way to get Garry's Mod on PC is through the Steam store for $9.99. Open the Steam Store. Search for "Garry's Mod" and add it to your cart.
If you specifically want the nostalgic "Garry's Mod 10" version, you must first own the retail game on Steam. Community members have archived these older versions for use as "Source mods".
Download the "Garry's Mod 10 Classic" files from community archives like Steam Community groups or ModDB .
: Minimum 2 GHz processor, 4 GB RAM, and 5 GB of storage space (20GB+ recommended).