The closest thing I can think of off the top of my head would be to create a transparent button object underneath all other layout objects to catch any taps that don't target some other interactive object. What are you trying to accomplish by detecting all taps?
I'm not sure how the OnLayoutKeystroke trigger is handled by filemaker GO, but that would be my guess.
The other solution would be to put an object (IE no fill text box) on the bottom of the layer stack that stretches the entire layout, and then attach an OnObjectEnter trigger to that object. This would fire an action anytime this text box was activated (by clicking anywhere outside of the rest of your layout objects).