Weidman would beat the crap out of GSP.
He's bigger, taller, longer, stronger. GSP can't get him down, and if Anderson had trouble figuring Weidman out standing, GSP isn't going to fare well either.
Weidman would knock him out standing or take him down and beat him to a pulp or sob him. He's basically a bigger, better GSP, only he can actually finish fights too.
"Do you believe you're still as fast as you were, even 10 years ago?" -Michael "The Voice" Schiavello
"Maybe faster..." Steven Seagal