Instructions
Open your Steam client by double-clicking the icon on your Windows task bar.
Type "GMOD" into the "Search the Store" box under the "Store" tab.
Click "Garry's Mod" and hit "Add to Cart" to put the mod in your Steam basket.
Look to the right of the shopping cart page and click "Half-Life 2" under the "Recommended" tab.
Click "Add to Cart" on the "Half-Life 2" page.
Click "Purchase for Myself" to download and install both applications. Once installation is complete, you can launch "Garry's Mod" from your Steam library by clicking "Library," clicking the "All Games" drop-down and clicking "Tools." Click "Garry's Mod" to run the program.