How to Host a Private GMod Server for Free and Fast

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.
VPS Hosting