Although thier hair is short a GSP has a very dense coat. After all the breed was created in a country where they have real winters. I would use a neoprene vest for waterfowling in your neck of the woods. But for upland birds the GSP coat will be fine.
I ride mine down the road in a dogbox in the back of a open pickup at high fuel consumption rates in the winter with no problems. Of course the GA winters pale in comparison to yours. So maybe a campershell for the truck or in the back of a SUV in kennel.