You could always change out the handlebars and there's a lot of choices out there. Here's a general handlebar guide for everyone. some of them are flatter and less curvy for mounting things like a GPS.
This is one way to go about it without having to worry about space although some will worry about how weird it might look, but when you have a real need for certain features, you have to let go of cosmetics to some extent:
Combining it with a vent visor like the above makes it even better. I would even consider tinting the visor to improve cosmetics, in that you get additional functionality.
Most people, including me, don't think about mounting it on top of the gauge since it's not a common method but now that you've brought it up people will have another method for GPS mounting. Just need to find that mount.
I think I'll wait for the adventure model instead of the base g310r. That one will most likely come with a wind screen and maybe a straighter handlebar for mounting things to if it takes after the 1200 GS in that aspect.
Touratech already has a GPS mount for the BMW R1200GS Adventure and they may come out with what you're looking for if you're going for the mini GS. they're pretty well known and it connects to the windscreen support bar.