That would be a fun fight to watch. One good leg kick from Barry could end it. GSP is much faster though, so he could potentially stay on the outside and out-point him. Could GSP take Barry down? That would be tough considering the size difference, but I wouldn't put it past GSP. I guess I would bet on Barry, simply because he would just need one good punch or kick to win, while GSP would have to avoid him for the entire fight and/or get takedowns to win.
this question is stupid, they have weight classes for a reason, I still don't think that Pat Barry is impressive in any way but GSP fights at 170 and Barry is what a heavyweight or light-heavyweight.