Enter the network name, password and click "Create".

Step 2: Ask your friends to join the network
Send your friends the name of the Radmin VPN network and the password to join to this network.

Step 3: Run Garry's mod

Step 4: Add the game to the firewall exceptions
You need to add rules to the firewall. Open Radmin VPN -> System -> Firewall Exceptions and add "gmad.exe" to the exceptions.

Step: 5 Game server configuration
Go to the main menu of Garry's mod and click "Start New Game".

Choose type of the game. Choose map.

On the right side of the screen, select the number of players.

Enter your server name. Select the "Peer-To-Peer" option. Click "Start Game".

Join Multiplayer Game

Step 1: Install Radmin VPN and connect to the network
Download and install Radmin VPN. In the main Radmin VPN window, click "Join Network". Enter the Network Name and Password and click "Join". 

Make sure you and your friends are on the same network.

Step 2: Run Garry's mod

Step 3: Add Rules to the Firewall
You need to add rules to the firewall. Open Radmin VPN -> System -> Firewall Exceptions and add "gmad.exe" to the exceptions.

Step 4: Join the game
Go to the main menu of Garry's mod and click "Options". Navigate to the "Keyboard" tab in the pop-up window that appears. At the bottom, select "Advanced". Check the "Enable developer console" box.

Open the "Developer Console" by pressing "~" on your keyboard (its the key below Esc) and type there the command to join "connect gameIP" (example: "connect 26.143.68.70").

Now you can enjoy the game with your friends.