Which GPS? The Zumos are MC-friendly and have the buttons on the left-side of the unit, so left mounting is best for these. I would think if you interact at all with the GPS, regardless of where the buttons are located, you'd want to use your left hand. OTOH it could be argued that interacting while riding is a bad idea.
One side or the other is preferable to the centre if you ride two up, then your passenger can see it, if they feel inclined. My wife keeps an eye on ours, especially if there is a lot of traffic, or an awkward turning coming up. Two pairs of eyes looking out! Mine is on the right because it least obscures other things. I have a TomTom which is touch screen, so it doesn't matter where it is to operate.
If you voted for Obama, mount it on the left. Seriously, why would you ask? Mount it wherever the hell you want to mount it. Why would you even ask this question? After a couple of Jumbo Margaritas at the local watering hole, I don't even care.