I think Toxic is right. The problem is Shields is the first guy to have the BJJ and the size/strength/wrestling to reverse GSP's top control on the ground. GSP might end up worried about someone's ground game, who knows how he'll do then.
Shields' BJJ is highly overrated. Who has he ever submitted worth noting?
Lawler, Daley, Thompson, Pyle, Pariente, Steinbeiss, Cooper, Jackson, Velarde, Ritch.
Those are the fighters he has submitted... Pyle is by far the best ground fighter Shields has submitted.
Shields has fought other decent ground guys but if you watch those fights will notice they are lay and pray clinics... He has ok takedowns, good BJJ, horrible stand up and weak ground and pound. He will not finish a fighter the quality of GSP and as such has to win a decision against a far better rounded fighter that has better conditioning.
GSP wins this fight however he wants but most likely he will pick Shields apart standing and tire him out with the odd takedown. Shields is a gamer and will be tough to finish but if he has another tough weight cut I could definitely see a tko win for GSP later in the fight.