Instructions
Connect to a server where players are currently playing on the Dustbowl map. You can view the current map in the server browser window which appears when you start a game.
Press the "Back" key on your controller to access the character selection screen. Select either the Soldier, the Demoman, the Pyro or the Sniper.
Move towards one of the gates, until you are standing against it. Then fire, using your primary weapon. Your projectiles will go right through the gate and injure members of the opposing team.