How to Create a Multi-Player Game With G-Modby Tim RaudUpdated September 22, 2017
G-Mod, or "Garry's Mod," is a sandbox-style computer game that runs on the Source physics engine. Players are allowed to create and manipulate objects from the "Half Life 2" games, as well as other Valve products. G-Mod has a single player and multiplayer mode. In multiplayer, players can work together, creating and sharing items to create much larger and interactive environments. If you create your own G-Mod server, you will have the ability to invite others to your server or remove them if you desire.
Load G-Mod through the game client Steam; you will be taken to the game's main menu.
Click "Create Multiplayer" to launch your own game server. Choose the server options in the dialog box that appears, and click "Launch" to start your server.
Press "Alt" plus "Tab" to go back to the Steam client. Open your friends list, and right-click the name of the friend; select "Invite Friend" to invite them to your server. If your friends can not join your server, continue.
Shut down your G-Mod server, and type your IP address in your web browser to open your router configuration. You can find your IP address by typing "ipconfig" in the "Command Prompt" window.
Click the "Port Forwarding" tab in your router configuration; you should see several rows of empty boxes.
Open the G-Mod ports by typing in these configurations of Protocol:
Inbound, Outbound, each in a separate row; "UDP: 1200, 1200," "UDP: 26901, 26901," "UDP: 27000, 27015," "TCP: 27020, 27039," "UDP: 27015, 27015," "UDP: 27020, 27020" and "TCP: 27015, 27015".
Save the new "Port Forwarding" settings, and close your router configuration. Relaunch your G-Mod server to begin playing multiplayer.