Instructions
Go to whatsmyip.org and write down your IP address. You will need this for others to connect to your server.
Open "Garry's Mod."
Click "Options," then "Keyboard" and then "Advanced." Check "Enable developer console." Anyone who wants to connect to the server will need to do this as well.
Click "Start New Game." Select a map and then click "Start Multiplayer."
Press the "~" key to bring up the developer's console. Type "sv_lan 0" then press "Enter." Then type "heartbeat" and press "Enter." These two commands will connect your server to Steam.
Invite a player to your game by pressing "Shift+Tab" to bring up Steam. Select them from your friends list, right-click on their name and select "invite to game." Players can also join by opening the developer console and typing "connect" and your IP address.