For GDB v2.0 device, there is a round hole with reset button inside. Use a pen or paper clip to hold the button for 15 seconds. You will see the LED light blink red every two seconds. Tap ‘+’ in ‘My Devices’ view in the GDB remote app to setup the device again.
For GDB v1.0, there is a button on the garagedoorbuddy box next to the power connector and ethernet connector port. To reset the password to factory default, hold the button down for 15 seconds.