No. GSP could even win standing up. Not that I think that's what's going to happen, but it would not surprise me either.
There is guys with better standup than bisping, guys with better wrestling, for sure better submission and that type of grappling.
How this fight plays out depends on GSP's shape. If he is as good as he was before having injury problems, then I don't think Bisping has much of a chance.
GSP will come in heavier than usual. Maybe he will be stronger, hit harder than before. Maybe he will be slower and tire out. Maybe he looks great at 185.
"The Old GSP", by that I mean the young GSP we used to see fight, was a lot more explosive and quicker than Bisping.
I think GSP is too proud to come back if he knows he will look like shit. On the other hand, maybe he think's it's worth it for a few million dollars, I don't know him personally but I don't think it's likely.