Creating your own Garry's Mod server gives you a place where you're in control. In addition, hosting one is easier than you might think. Every copy of Garry's Mod comes with a built-in server that you can activate in just a few minutes of set-up. In this server, you're the administrator, and can make sure that everyone has a good time, without disrupting the work of others. Best of all, it's free and easy.
Start Garry's Mod and pick "Start New Game" from the main menu.
Choose the map you want to play and click the "Play Multiplayer" button on the bottom-right of the screen. In addition to maps you already have, you can choose new maps by clicking on the "Toybox" tab at the top of the window and choosing new maps.
Invite friends by pressing the Shift and Tab keys at the same time to bring up the Friends window, right-clicking on the person to invite and choosing "Invite to Game" from the drop-down menu.
You can kick and ban people from your server using the "Kick" and "Ban" commands from the console.
Don't overload your computer or your network connection by allowing more than a few people to join your server.