Instructions
Download Steam, a gaming platform, then run and install it by double clicking on the installer *.msi file. Log onto Steam and download "Half-Life 2" and "Garry's Mod."
Open the Steam Client, select "Games," then double-click on "Garry's Mod." Click on "Play Singleplayer" on the Main Menu.
Click on the "Icons" tab, then scroll down to "Half Life 2" and click on the "+" symbol to expand the maps for "Half Life 2." Double-click on the first map to start the "Half-Life 2" campaign. Once completing a section or a level, the next level in sequence will load, as normal.