First, navigate to the event that you want to do a gun start on. Then navigate to the RFID tab on the sidebar on the left.
Towards the middle of the screen, you will see a section for "Manage Events." Choose the correct event and below you will see the option for "Fire Gun" show up. Click on that when you are ready to fire the gun and start your race.
Once your athletes have crossed the finish line, hit "Finish" next to where it says "Official Time" to stop the timer.