Obviously the gp2x interface is quite different from the normal mouse+monitor+keyboard combination normally used to play with Pd patches. To navigate between GUI elements use the joystick. If you want to modify a GUI element hold down the 'B' button. If the widget is a toggle, it will toggle when you press 'B'. If it's a numberbox you must hold down 'B' and then move the joystick to change the number. Left/right will modify it by one, whilst up/down will modify the value by 12.

To exit the patch and go back to the gp2x menu hold down Start and Select at the same time.