I'll never question GSP's desire to be the best, or his training, preparation, and focus to that end. For me, if anyone can avoid Shield's takedowns for 5 rounds, it's GSP. But likewise, I think if anyone can take GSP down and control him for 3 out of 5 rounds, it's Shields. Jake clearly isn't going to pound Georges out with his pillow-hands, and it's unlikely he's going to catch him in a submission. But I could definitely see Jake out-position and control him on the mat for a round or three.
I have to think Georges' gameplan is to avoid the mat at all costs in this. If he takes Jake down, I see him getting swept and mounted (sorry Danaher!). Shields is a very, very, very different kind of grappler than Kos or Fitch. I might be wrong, but I think he'll be the biggest, strongest WW that Georges has ever fought and definitely the best grappler (i.e. wrestling AND jj).