This is a great idea for a Ram Ball mount, but (IMO) not so great of a location for a GPS. You'll need almost super human self control not to end up with your head down and locked with a GPS mounted that low. Yeah I know, you never take your eyes off the road for more than split second...but eventually you will. Then when you get away with it, you'll do it more and more and eventually something will happen ahead of you that you'll miss seeing, if you're lucky you'll only scare yourself when you do, if not.....
In my opinion a GPS with all it's features can be more distracting than a cell phone and it needs to be much higher. If you can't devise a way to get it high in the center, it's much safer mounted high on the handlebars either left or right of center than down low in the center.
We all have opinions, and now you know mine...just pretend I'm max' typing about driving with your high beams on during the day.