This idea is predicated on the basis that he will beat GSP. Which he won't. He's too one dimensional. GSP is the most well rounded fighter in the game. You know what Shields gameplan is every time he steps into the cage. Go for the takedown, tire you out, and submit you.
Problem is he won't be able to get the takedown against GSP. The only time GSP has spent on his back is against Kos who has an extremely explosive and hard to stop double leg. Shields takedowns have been stuffed by Paul Daley. Even if he gets GSP down I don't believe he will be able to hold him down or inflict any significant damage.
This fight will go like any fight where you have two excellent grapplers. It will be a standing fight and GSP will win that easily because Shield's stand up is absolute garbage. Not Ben Askren level bad but what is?
That is not dead which can eternal lie and withs strange aeons even death may die.