Instructions
Launch GMod by double-clicking on "Garry's Mod" in your Steam library. Steam is the application from Valve that you used to purchase and install GMod.
Click on "Create multiplayer" in GMod's main menu.
Select the game that contains the map you want to use from the drop-down menu at the top of the window. Then click on the map you want to play on.
Click on the "Create game" button. Your friends will now be able to see your game in the server browser window inside GMod.