I don't know. Good question. I don't think Bisping is really a great fighter, but I figured he'd beat GSP due to the size issue, style, and layoff. Style wise, Bisping should have had it much easier with GSP than Gastelum. I gave him no chance against Gastelum since I saw Gastelum destroy Kennedy, a guy who outclassed Bisping more handily than people are willing to admit.
GSP wasn't known for power in his hands, had a layoff, was fighting with more muscle, and liked to wrestle, things which would seemingly hurt against a guy like Bisping, who has insane cardio and is hard to wrestle.
Gastelum is known for big power and didn't go to 185 looking like he'd gas on his way to the cage. I gave Bisping no chance.