Get "Garry's Mod" on Steam by signing up for a Steam account, purchasing the game and then downloading it. "Garry's Mod" is a physics sandbox game that does not have any goals or gameplay rules.
Click Login on the top right to create a Steam account. Fill in the registration form, agree to the agreement, and click Create my Account. Once you have an account, click the green Install Steam button on the top right to download the application. Make sure to run the installer as an administrator.
Once it's installed, launch Steam, and log into your account. Type "Garry's Mod" on the search box, and click the game on the list. Once on the game's page, click Add To Cart next to the price, click Purchase For Myself, and provide payment information. After your bank confirms the payment, click Install Games to install "Garry's Mod."
If the installation does not begin, click Library on the top, select Games, and click "Garry's Mod" from the list on the left. Click the blue Install button on the right. Steam then displays the download progress as a bar at the bottom. Click on the bar to see the remaining time for the download.
The developer warns that you may need to purchase additional games, such as Team Fortress 2, to access some game modes and servers. All required games are available on Steam.