GSP didn't beat people based on strength; it was his explosiveness and technique. Also, he was generally fighting guys his size or smaller. It will be more difficult to employ his technique against Bisping, who is much bigger than him. And the explosiveness that he once had was obviously fading when he left the sport; there's no reason to believe it will magically reappear now after 4 years off.
Is it possible that GSP pulls a Couture-Sylvia type of upset? Possibly. After all, Bisping is the most beatable MW champion we've had in over a decade. But I wouldn't bet on it. I'd be a little surprised if GSP pulled this one out.