Strike with him a bit to make him think that's the plan, then take him down and beat the crap out of him from half guard and side control. Gas him out on the ground, beat him up in the standup some more, then take him back down for more ground & pound. Repeat till the fight is stopped or the 25 minutes are up.
My first thought was like everyone else, that GSP would keep it standing and outstrike Sheilds, but after I thought it over I've changed my mind, something like the above is exactly what is going to happen.
The Jackson camp always gameplans extremely well, and they know very well Sheilds will want to take the fight to the ground, and expects GSP to try and stand. We saw what happens when you surprise someone thinking like that in the Penn-Fitch fight. Fitch himself said he was shocked by Penns aggressiveness taking him down and wasn't ready for it.
Sheilds won't be either, and GSP can easily take him down, and maintain top control. He will not be concerned about Sheilds subbing him, nor should he be. Sheilds is good at BJJ, very good, but if you go back and look at his submission wins, almost all of them are from the top, against guys who suck hard on the ground.
Sheilds will be on the bottom, and GSP is not exactly easy to submit. Sheilds last two subs were Paul Daley and Robbie Lawlor, both are utterly useless on the ground. He couldn't sub Henderson with 4 rounds on top, was literally 2 seconds away from getting choked out by Miller, and should have lost to that ground wizard Kampmann.
GSP can and will outstrike Sheilds, but he won't be afraid to take Sheilds down, and he shouldn't be. If anyone gets subbed, it will be Sheilds.