How to Reinstall "Garry's Mod"by Seth AmeryUpdated September 22, 2017
Available only on Steam, "Garry's Mod" stays registered to your account when you purchase or redeem a gift copy of the game. This means even after you delete the files from your system, you can download and install it again at any time. You will need the Steam gaming software downloaded to reinstall "Garry's Mod."
Reinstalling Steam Games
Open your Steam client and sign in to the account with "Garry's Mod" registered to it. If you don't have the software installed, you can acquire it from its website (see Resources). Within the Steam client, move the cursor over the Library tab, selecting "Games" from the drop-down menu that appears. Right-click "Garry's Mod" in the list that appears; if it's uninstalled, it'll appear in a dim grey font rather than light grey. Select "Install Game" to begin the process. Alternatively, you can left-click the game title followed by the "Install" button in Steam's main pane.